Antagonistic mongrel bit policeman (12)
I believe the answer is:
incompatible
'antagonistic' is the definition.
(I've seen this before)
'mongrel bit policeman' is the wordplay.
'mongrel' indicates anagramming the letters (I've seen this in other clues).
'bit'+'policeman'='bitpoliceman'
'bitpoliceman' is an anagram of 'INCOMPATIBLE'.
(Other definitions for incompatible that I've seen before include "Unable to get on" , "Not mutually suited" , "Not going together" , "Not able to be used together" , "Ill-matched" .)