Belly could be advantage eating everything (7)
I believe the answer is:
balloon
'belly' is the definition.
(thesaurus)
'advantage eating everything' is the wordplay.
'advantage' becomes 'boon' (both can mean a benefit or asset).
'eating' indicates putting letters inside.
'everything' becomes 'all' (all things).
'boon' enclosing 'all' is 'BALLOON'.
'could be' acts as a link.
(Other definitions for balloon that I've seen before include "Inflate" , "plaything" , "Swell, increase rapidly" , "Inflatable rubber bag, could be filled with hot air" , "One blown up for party" .)