group which deactivates the benzene ring  towards electrophilic substitution but which directs the incoming group principally to the O-Positions is 

-Cl group is o-, p-directing due to +R effect ; however it is deactivating due to strong —I effect of Cl (difference from other o-, p-directing groups which are activating). The net result is that chlorobenzene undergoes o-, p-substitution, but with difficulty