Snack one's left to boil over (5)
I believe the answer is:
donut
'snack' is the definition.
'one's left to boil over' is the wordplay.
'left' indicates named letters should be taken away.
'to boil over' becomes 'doonesnut' (both can mean to become very angry).
'doonesnut' with 'ones' removed is 'DONUT'.
(Other definitions for donut that I've seen before include "ring coming across the Atlantic" , "treat for Homer Simpson" , "something to eat" , "American pastry" , "Fast food snack" .)