As keen as mustard backing female singer (4)
I believe the answer is:
avid
'as keen as mustard' is the definition.
I can't tell whether this defines the answer.
'backing female singer' is the wordplay.
'backing' says the letters should be written backwards.
'female singer' becomes 'diva' (I've seen this before).
'diva' back-to-front is 'AVID'.
Can you help me to learn more?
(Other definitions for avid that I've seen before include "passionate" , "Greedy like diva" , "Keenly interested" , "Keenly enthusiastic" , "Ardent about diva" .)