Shock as bits of food served up (4)
I believe the answer is:
'shock' is the definition.
(both can mean to surprise)
'bits of food served up' is the wordplay.
'bits of' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'food' becomes 'nut' (type of food).
'served up' says the letters should be written in reverse (in a down clue, going upwards).
'nut' written backwards gives 'tun'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for stun that I've seen before include "Daze, scatter senses" , "Render temporarily senseless" , "Nuts (anag)" , "Stupefy - daze" , "Render semi-conscious with blow" .)