Singer, performing as well as ever, about to make a comeback (8)
I believe the answer is:
informer
'singer' is the definition.
(sing can mean to inform or talk to the police)
'performing as well as ever about to make a comeback' is the wordplay.
'performing as well as ever' becomes 'inform' (someone in form is at their usual standard).
'about to' becomes 're' (I've seen this in other clues).
'make a comeback' says the letters should be written backwards (I've seen 'making a comeback' mean this).
're' backwards is 'er'.
'inform'+'er'='INFORMER'
(Other definitions for informer that I've seen before include "spy" , "stoolie" , "A grass, one who tells all to the police" , "One aiding police" , "One telling" .)