All the cops in new uniform? Wrong! (5,7)
I believe the answer is:
plain clothes
'uniform? wrong' is the definition.
(I've seen this before)
'all the cops in new' is the wordplay.
'new' indicates an anagram.
'all'+'the'+'cops'+'in'='allthecopsin'
'allthecopsin' anagrammed gives 'PLAIN-CLOTHES'.
(Other definitions for plain clothes that I've seen before include "sort of detective" , "Act of bringing together (sides in dispute)" , "uniform? Wrong" , "Mufti" , "Policeman's non-uniform wear" .)