Turn informer among convicts in gaol (4)
I believe the answer is:
sing
'turn informer' is the definition.
(I've seen this before)
'among convicts in gaol' is the wordplay.
'among' indicates a hidden word.
'SING' is hidden in the letters of 'convicts in gaol'.
(Other definitions for sing that I've seen before include "Turn informer" , "Croon" , "Confess" , "Inform - carol" , "Chorus" .)