Gamble about large boxing prize (4)
I believe the answer is:
belt
'boxing prize' is the definition.
'gamble about large' is the wordplay.
'gamble' becomes 'bet' (betting is a kind of gambling**).
'about' means one lot of letters goes inside another.
'large' becomes 'l'.
'bet' placed around 'l' is 'BELT'.
(Other definitions for belt that I've seen before include "Race" , "Zone - connecting strip" , "drive at speed" , "District" , "sock or > tie" .)