Bit of breakfast undercooked served up with crackers (4)
I believe the answer is:
nuts
'crackers' is the definition.
('nuts' can be a synonym of 'crackers')
'bit of breakfast undercooked served up' is the wordplay.
'bit of' says the answer is hidden in the clue.
'served up' says the letters should be written in reverse.
'stun' is hidden within 'breakfast undercooked'.
'stun' written backwards gives 'NUTS'.
'with' is the link.
(Other definitions for nuts that I've seen before include "Hard-shelled fruit" , "Crazy (sl.)" , "Beans" , "Barmy (colloq)" , "Metal fasteners" .)