Inform on mixing gins (4)
I believe the answer is:
sing
'inform on' is the definition.
(thesaurus)
'mixing gins' is the wordplay.
'mixing' indicates an anagram.
'gins' is an anagram of 'SING'.
(Other definitions for sing that I've seen before include "Chant" , "Turn informer" , "Confess" , "Croon" , "Perform (a song)" .)