KO upset cuckoo! (4)
I believe the answer is:
stun
'ko' is the definition.
(thesaurus)
'upset cuckoo' is the wordplay.
'upset' is a reversal indicator.
'cuckoo' becomes 'nuts' (I've seen this in another clue).
'nuts' written backwards gives 'STUN'.
(Other definitions for stun that I've seen before include "Knock unconscious" , "Shock or astonish" , "'Shock, daze (4)'" , "Shock into silence" , "Daze, render semi-conscious" .)