Antagonistic policeman bit criminal (12)
I believe the answer is:
incompatible
'antagonistic' is the definition.
(I've seen this before)
'policeman bit criminal' is the wordplay.
'criminal' is an anagram indicator.
'policeman'+'bit'='policemanbit'
'policemanbit' with letters rearranged gives 'INCOMPATIBLE'.
(Other definitions for incompatible that I've seen before include "Unsuited - contrary" , "Not mutually suited" , "Not usable together" , "Not able to be used together" , "Not able to go together" .)