Amaze by juggling nuts (4)
I believe the answer is:
stun
'amaze' is the definition.
(to stun is to amaze or shock)
'juggling nuts' is the wordplay.
'juggling' is an anagram indicator (I've seen this in other clues).
'nuts' anagrammed gives 'STUN'.
'by' is the link.
(Other definitions for stun that I've seen before include "Amaze; knock senseless" , "Render temporarily senseless" , "Render insensitive" , "Daze, scatter senses" , "Confound" .)