We're told 'cock', perhaps, is offensive (4)
I believe the answer is:
foul
'offensive' is the definition.
('foul' can be similar in meaning to 'offensive')
'we're told cock perhaps' is the wordplay.
'we're told' indicates a 'sounds like' (homophone) clue.
'cock perhaps' becomes 'fowl' (cock is a kind of fowl).
'fowl' sounds like 'FOUL'.
'is' is the link.
(Other definitions for foul that I've seen before include "Having offensive smell or taste" , "Illegitimate tackle" , "Repulsive" , "Disgusting" , "A breach of sporting rules" .)