Keen opera star to make a comeback (4)
I believe the answer is:
avid
'keen' is the definition.
(I've seen this before)
'opera star to make a comeback' is the wordplay.
'opera star' becomes 'diva' (diva can mean a female opera singer).
'to make a comeback' says the letters should be written backwards (I've seen 'making a comeback' mean this).
'diva' backwards is 'AVID'.
(Other definitions for avid that I've seen before include "Fanatical" , "Extremely keen" , "Greedy - enthusiastic" , "Extremely enthusiastic" , "'Eager, zealous (4)'" .)