Not quite a fat lady (3)
I believe the answer is:
sue
'lady' is the definition.
'not quite a fat' is the wordplay.
'not quite' means to remove the last letter.
'a fat' becomes 'suet' (suet is a kind of fat).
'suet' with its last letter taken away is 'SUE'.
(Other definitions for sue that I've seen before include "Take legal case against" , "Start proceedings against (someone)" , "take to 10 [COURT] !" , "Bring charges against - girl" , "Humbly request" .)