Reactivity of chlorobenzene and benzene in electrophilic substitutions

Electrophilic substitution relies on the electrophile being attracted to the delocalised electrons in the benzene ring. But chlorine is more electronegative than carbon, and so draws electrons in the ring towards itself.

That makes the electron density around the ring rather less in chlorobenzene. It is less attractive for electrophiles, and so the reaction is slower.

considering electrophilic aromatic substitution is chlorobenzene or benzene more reactive?

Let's expand the question to include all of the halobenzenes.

All of the halogens ($\ce{F, Cl, Br, I}$) are more electronegative than hydrogen. Therefore they will inductively remove electron density from a benzene ring. However, you can also draw resonance structures for the various halobenzenes where the halogen substituent is donating electron density to the ortho and para positions of the benzene ring through resonance.

Therefore, in the case of the halobenzenes these two effects are working in opposite directions. In order to determine which effect controls the reaction, we need some data.

The following table compares the relative rates of electrophilic aromatic nitration of the halobenzenes to the rate for benzene itself. If we assign benzene a relative rate of 1, we see that all of the halobenzenes react slower than benzene itself. This tells us that the inductive effect is stronger than the resonance effect in the halobenzene series.
\begin{array} \hline \text{Relative rates of aromatic electrophilic nitration} \\ \hline \end{array} \begin{array}{|c|c|c|c|} \hline \ce{Ar-X} & \text{Relative rate} \\ \hline \ce{Ar-H} & 1.0 \\ \hline \ce{Ar-F} & 0.11 \\ \hline \ce{Ar-Cl} & 0.02 \\ \hline \ce{Ar-Br} & 0.06 \\ \hline \ce{Ar-I} & 0.13 \\ \hline \end{array}

(data source: see p. 26 here)

However, when the halobenzenes do undergo electrophilic aromatic substitution, they react preferentially at the ortho and para positions, because, as explained above, the resonance effect donates electron density to the ortho and para positions. Therefore, these two positions are the least deactivated and react preferentially.

Since chlorine is an electron withdrawing group, so it withdraws electron from aromatic ring. As a result of which the electron density of the aromatic ring decreases and deactivates the ring. Chlorobenzene is less reactive than benzene towards electrophilic substitution reaction. As halogen atoms make the aromatic ring less reactive towards the upcoming electrophile, they are regarded as deactivating groups but they act as ortho para deactivators due to the development of partial negative charge on ortho and para postion. Hence chlorobenzene is ortho para deactivator and is less reactive than benzene.

Trying to make it easy: electrophile means electron loving so in haloarenes the halogen group pulls electron from benzene. But electrophiles need electrons so benzene is more active.

Chlorobenzene is less reactive than benzene towards electrophilic substitution reaction due to - I effect.As chlorine is electron withdrawing group it deactivates the benzene ring and reduces the electron density on benzene ring and hence make the aromatic ring less reactive towards the upcoming electrophile.

$\ce{Cl}$ is a weak ring deactivating group. This means chlorobenzene is less reactive towards electrophiles than benzene. But, due to the presence of non-bonding electrons, they can donate electron density through formation of $\pi$-bonds.

$\ce{Cl-}$ is still an ortho/para directing group.

As you have said the chlorobenzene is deactivating -This should be a bit of a foreshadowing.

The chloro substituent will function to withdraw electrons from the benzene ring and reduce its electron density, this will make it less reactive with electrophiles and reduce the reaction rate.