Passionate prima donna in the ascendant (4)
I believe the answer is:
avid
'passionate' is the definition.
(I've seen this before)
'prima donna in the ascendant' is the wordplay.
'prima donna' becomes 'diva' ('diva' can be a synonym of 'prima donna').
'in the ascendant' shows that the letters should be reversed in order.
'diva' back-to-front is 'AVID'.
(Other definitions for avid that I've seen before include "Greedy like diva" , "Showing active interest and enthusiasm" , "Mad for diva" , "Keen or greedy" , "passionate" .)