KO — crazy to get up (4)
I believe the answer is:
stun
'ko' is the definition.
(thesaurus)
'crazy to get up' is the wordplay.
'crazy' becomes 'nuts' (nuts can informally mean crazy or mad).
'to get up' is a reversal indicator.
'nuts' reversed gives 'STUN'.
(Other definitions for stun that I've seen before include "Concuss, stupefy" , "Render temporarily senseless" , "Daze, paralyse" , "Daze, overwhelm" , "Daze or bewilder" .)