Attack dog kept by hitmen occasionally (6)
I believe the answer is:
impugn
'attack' is the definition.
(I've seen this before)
'dog kept by hitmen occasionally' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog).
'kept by' is an insertion indicator.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternate letters of 'hitmen' are 'imn'.
'pug' placed within 'imn' is 'IMPUGN'.
(Other definitions for impugn that I've seen before include "Try to discredit, express doubts about honesty" , "Call into question; criticise" , "Attack as false, challenge" , "Attack verbally" , "Express doubts about truth or honesty of, say motives" .)