Very keen to make a comeback as a singer (4)
I believe the answer is:
diva
'a singer' is the definition.
(diva can mean a female opera singer)
'very keen to make a comeback' is the wordplay.
'very keen' becomes 'avid' (I've seen this before).
'to make a comeback' says the letters should be written in reverse.
'avid' reversed gives 'DIVA'.
'as' acts as a link.
(Other definitions for diva that I've seen before include "Female opera star" , "Avid like opera singer" , "Famous female singer" , "Top opera singer" , "High-maintenance opera singer?" .)