Sees chest heaving, having eaten undercooked snack (6,6)
I believe the answer is:
cheese straws
'snack' is the definition.
'sees chest heaving having eaten undercooked' is the wordplay.
'heaving' is an anagram indicator (letters thrown or heaved around).
'having eaten' indicates putting letters inside.
'undercooked' becomes 'raw'.
'sees'+'chest'='seeschest'
'seeschest' is an anagram of 'cheesests'.
'cheesests' enclosing 'raw' is 'CHEESE STRAWS'.
(Other definitions for cheese straws that I've seen before include "Twig-shaped nibbles" , "Savoury snacks" .)