Singer doing well, joining Queen perhaps (9)
I believe the answer is:
informant
'singer' is the definition.
(sing can mean to talk to the police)
'doing well joining queen perhaps' is the wordplay.
'doing well' becomes 'in form'.
'joining' means one lot of letters go next to another.
'queen perhaps' becomes 'ant' (queen is a type of ant).
'inform'+'ant'='INFORMANT'
(Other definitions for informant that I've seen before include "Traitor" , "Sneak" , "source of dope" , "One who sings" , "Snitch" .)