Gunner apt to become offensive (9)
I believe the answer is:
repugnant
'offensive' is the definition.
('repugnant' can be similar in meaning to 'offensive')
'gunner apt to become' is the wordplay.
'to become' indicates an anagram.
'gunner'+'apt'='gunnerapt'
'gunnerapt' is an anagram of 'REPUGNANT'.
(Other definitions for repugnant that I've seen before include "revolting" , "Offensive to the mind" , "Abhorrent" , "Loathsome" , "Nasty" .)