Mounted enthusiastic female singer (4)
I believe the answer is:
'female singer' is the definition.
(term for a famous opera singer)
'mounted enthusiastic' is the wordplay.
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
'enthusiastic' becomes 'avid' (similar in meaning).
'avid' reversed gives 'DIVA'.
(Other definitions for diva that I've seen before include "Prima dona" , "Callas, say" , "Avid like opera singer" , "Long narrow top" , "Operatic female singer" .)