Playing well, accompanying Queen's singer? (8)
I believe the answer is:
informer
'singer?' is the definition.
(sing can mean to inform or talk to the police)
'playing well accompanying queen's' is the wordplay.
'playing well' becomes 'inform' (to be 'in form' is to be at one's best in a sport).
'accompanying' is a charade indicator (letters next to each other).
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'inform'+'er'='INFORMER'
(Other definitions for informer that I've seen before include "Telltale" , "who may betray someone?" , "stoolie" , "Whistle-blower" , "One snitching to police" .)