Turn informer twice in NY prison (4,4)
I believe the answer is:
sing sing
'ny prison' is the definition.
(I've seen this before)
'turn informer twice' is the wordplay.
'turn informer' becomes 'sing' (I've seen this before).
'twice' indicates the doubling of some letters.
'sing' doubled is 'SING-SING'.
'in' is the link.
(Other definitions for sing sing that I've seen before include "NY prison" , "New York state prison" .)