Hit very hard (6)
'hit very hard' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for wallop that I've seen before include "Beat" , "Hit - the booze?" , "big hit" , "buffet?" , "Alcoholic drink" .)
(Other definitions for belt that I've seen before include "District" , "Zone; go quickly" , "Waist band; hit very hard" , "Swipe; item of clothing" , "See 18" .)