Backing singer's enthusiastic (4)
I believe the answer is:
avid
'enthusiastic' is the definition.
(similar in meaning)
'backing singer's' is the wordplay.
'backing' says the letters should be written in reverse.
'singer' becomes 'diva' (diva is a kind of singer).
'diva' written backwards gives 'AVID'.
(Other definitions for avid that I've seen before include "Fanatical" , "deeply enthusiastic" , "Greedy or eagerly desirous" , "Keen and eager" , "hungry" .)