Turn crazy and stupefy (4)
I believe the answer is:
stun
'stupefy' is the definition.
(I know that stupefy can be written as stun)
'turn crazy' is the wordplay.
'turn' shows that the letters should be reversed in order.
'crazy' becomes 'nuts' (nuts can informally mean crazy or mad).
'nuts' reversed gives 'STUN'.
'and' acts as a link.
(Other definitions for stun that I've seen before include "Astound; render senseless" , "what a cracker may do?" , "Leave speechless" , "Numb with nuts" , "Amaze; knock senseless" .)