Wager about large band (4)
I believe the answer is:
belt
'band' is the definition.
(I've seen this before)
'wager about large' is the wordplay.
'wager' becomes 'bet' (synonyms).
'about' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'bet' placed around 'l' is 'BELT'.
(Other definitions for belt that I've seen before include "Waist band; hit very hard" , "Zone -- thrash" , "Below this, a boxing blow is illegal" , "Sign of rank in karate" , "Zone; leather strip" .)