Very keen prima donna does a turn (4)
I believe the answer is:
avid
'very keen' is the definition.
(I've seen this before)
'prima donna does a turn' is the wordplay.
'prima donna does' becomes 'div' (I can't explain this - if you can you should believe this answer much more).
'turn' is a reversal indicator.
'div'+'a'='diva'
'diva' reversed gives 'AVID'.
Can you help me to learn more?
(Other definitions for avid that I've seen before include "'Eager, zealous (4)'" , "Burning" , "Very keen, desirous" , "Keen and eager" , "Keen, enthusiastic" .)