Belt holding in a belly (6)
I believe the answer is:
paunch
'belly' is the definition.
(I know that belly can be written as paunch)
'belt holding in a' is the wordplay.
'belt' becomes 'punch' (as verb - to belt someone).
'holding in' is an insertion indicator.
'punch' enclosing 'a' is 'PAUNCH'.
(Other definitions for paunch that I've seen before include "Protuberant belly" , "Gut" , "Beer belly" , "Large stomach or belly" , "Protruding belly" .)