Only one to turn informer twice (9)
I believe the answer is:
singleton
'only one' is the definition.
(I've seen this before)
'turn informer twice' is the wordplay.
'turn informer' becomes 'sing' (I've seen this before).
'twice' becomes 'leton' (I can't explain this - if you can you should give a lot more credence to this answer).
'sing'+'leton'='SINGLETON'
'to' is the link.
Can you help me to learn more?
(Other definitions for singleton that I've seen before include "In bridge, a holding of one card only in a suit" , "one diamond, perhaps" , "One object only" , "one without a mate?" , "Card that is only one held in its suit" .)