Singer very keen to retire (4)
I believe the answer is:
diva
'singer' is the definition.
(diva is a kind of singer)
'very keen to retire' is the wordplay.
'very keen' becomes 'avid' (I've seen this before).
'to retire' shows that the letters should be reversed in order.
'avid' in reverse letter order is 'DIVA'.
(Other definitions for diva that I've seen before include "demanding woman" , "Great performer" , "Operatic female singer" , "One who sings" , "For example Sutherland" .)