Certain to eat first of food with it -- a bellyful (7)
I believe the answer is:
surfeit
'a bellyful' is the definition.
(thesaurus)
'certain to eat first of food with it' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to eat' indicates putting letters inside.
'first of food' becomes 'f' (1st letter of 'food').
'with' says to put letters next to each other.
'sure' enclosing 'f' is 'surfe'.
'surfe'+'it'='SURFEIT'
(Other definitions for surfeit that I've seen before include "Excess, glut" , "Overindulge" , "Overindulgence" , "having had quite enough" , "Superabundance" .)