Stupefy with crazy return (4)
I believe the answer is:
stun
'stupefy' is the definition.
(I know that stupefy can be written as stun)
'crazy return' is the wordplay.
'crazy' becomes 'nuts' (nuts can informally mean crazy or mad).
'return' says the letters should be written in reverse.
'nuts' backwards is 'STUN'.
'with' is the link.
(Other definitions for stun that I've seen before include "Make dazed or numb" , "Knock into a semi-conscious state" , "Confuse" , "Daze, paralyse" , "Knock cold" .)