Dear,

p-nitrochloro benzene is more reactive towards nucleophilic substitution reaction as compared to chlorobenzene. In chlorobenzene, the lone pair on Cl is delocalized with PI electrons of ring. Whereas in p-nitro chloro benzene, presence of strong EWG i.e. NO ₂ further delocalizes the negative charge. As we know, more is the delocalization, more is the resonance and stable will be the intermediate formed and hence more is the rate of reaction.

Hope this helps!