Astound by juggling nuts (4)
I believe the answer is:
stun
'astound' is the definition.
(I've seen this in another clue)
'juggling nuts' is the wordplay.
'juggling' indicates anagramming the letters (I've seen this in other clues).
'nuts' anagrammed gives 'STUN'.
'by' acts as a link.
(Other definitions for stun that I've seen before include "cause amazement" , "deliver a knock-out blow" , "Render unconscious" , "Knock out temporarily" , "Confuse" .)