Chlorobenzene is o, p-directing in electrophilic substitution reaction. The directing influence is explained by

Halogens are moderately deactivating  because of their strong -I effect, overall electron density on Benzene ring decreases. It makes further substitution difficult. How ever, due to Resonance(+M) the electron density on o- and P- positions is greater than that at the m-position. Hence they are o- and p- directing groups.