Confirm half of bets on a heavy defeat (4,3)
I believe the answer is:
bear out
'confirm' is the definition.
(I've seen this before)
'half of bets on a heavy defeat' is the wordplay.
'half of' suggests halving the letters.
'on' means one lot of letters go next to another.
'heavy defeat' becomes 'rout' (I've seen this before).
'bets' halved is 'be'.
'be'+'a'+'rout'='BEAR OUT'
(Other definitions for bear out that I've seen before include "Confirm - or carry away" , "Corroborate, support with evidence" , "Confirm - or carry from house" , "Confirm (something)" .)