Amaze by cracking nuts (4)
I believe the answer is:
stun
'amaze' is the definition.
(I've seen this before)
'cracking nuts' is the wordplay.
'cracking' indicates an anagram.
'nuts' anagrammed gives 'STUN'.
'by' is the link.
(Other definitions for stun that I've seen before include "Astound; render senseless" , "Bowl over" , "Knock into a semi-conscious state" , "Anaesthetise" , "Daze, concuss" .)