Person who bets (6)
I believe the answer is:
punter
'person who bets' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for punter that I've seen before include "Better - rugby player - river boatman" , "boater" , "Person who bets" , "Professional gambler; client (inf.)" , "A colloquial better or a regular client" .)