Backing singer had moan (7)
I believe the answer is:
ululate
'moan' is the definition.
(thesaurus)
'backing singer had' is the wordplay.
'backing' says the letters should be written backwards.
'singer' becomes 'lulu' (I've seen this before**).
'had' becomes 'ate' (e.g. I had an ice cream).
'lulu' written backwards gives 'ulul'.
'ulul'+'ate'='ULULATE'
(Other definitions for ululate that I've seen before include "Wail in grief" , "Give a hoot" , "Emit long loud cries" , "Keen" , "Howl loudly" .)