Not all convicts in gaol turn informer (4)
I believe the answer is:
sing
'turn informer' is the definition.
(I've seen this before)
'not all convicts in gaol' is the wordplay.
'not all' indicates the answer is hidden within the clue.
'SING' is hidden within 'convicts in gaol'.
(Other definitions for sing that I've seen before include "Chant" , "Act as informer" , "Croon" , "serenade" , "Blow the whistle" .)