She's a knockout! (7)
I believe the answer is:
stunner
This clue might be a double definition.
'she's' is the first definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?
'a knockout' is the second definition.
('stunner' can be a synonym of 'knockout')
Can you help me to learn more?
(Other definitions for stunner that I've seen before include "Powerful punch" , "A great beauty" , "Peach" , "Good-looker (informal)" , "Amazing turn of events (colloq.)" .)