Pitch of backing singer (3)
I believe the answer is:
tar
'pitch' is the definition.
(synonyms)
'backing singer' is the wordplay.
'backing' says the letters should be written in reverse.
'singer' becomes 'rat' (both can mean an informant).
'rat' written backwards gives 'TAR'.
'of' acts as a link.
(Other definitions for tar that I've seen before include "Slang sailor or road surfacing material" , "Sailor - laid on road" , "Old salt, sailor" , "Familiar sailor - what a rat" , "Sticky black stuff" .)