Look a knockout! (4)
I believe the answer is:
stun
'look' is the definition.
The answer and definition can be both to do with perception as well as being verbs in their base form.
Maybe you can see an association between them that I can't see?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for stun that I've seen before include "Nuts (anag)" , "Knock cold" , "Knock into a semi-conscious state" , "Dumbfound" , "Daze, stupify" .)