Attractive person quietly leaving knockout match (5)
I believe the answer is:
cutie
'attractive person' is the definition.
(I've seen this before)
'quietly leaving knockout match' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'leaving' suggests deleting specific letters.
'knockout match' becomes 'cuptie' (I've seen this before).
'cuptie' with 'p' taken away is 'CUTIE'.
(Other definitions for cutie that I've seen before include "beauty" , "considered by most to be attractive" , "Smart girl" , "Smasher" , "something pleasing" .)