Shock when crackers pulled back (4)
I believe the answer is:
stun
'shock' is the definition.
(both can mean to surprise)
'crackers pulled back' is the wordplay.
'crackers' becomes 'nuts' (synonyms).
'pulled back' says the letters should be written backwards.
'nuts' reversed gives 'STUN'.
'when' is the link.
(Other definitions for stun that I've seen before include "Shock into silence" , "Put out for the count" , "Render unconscious" , "Confuse" , "Knock cold" .)