Enthusiastic singer needing backing (4)
I believe the answer is:
avid
'enthusiastic' is the definition.
(similar in meaning)
'singer needing backing' is the wordplay.
'singer' becomes 'diva' (diva can mean a female opera singer).
'needing backing' 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 "Keen and eager" , "Showing a very keen interest in something" , "passionate" , "Extremely enthusiastic" , "Greedily eager" .)