About to bet on a race (5)
I believe the answer is:
relay
'a race' is the definition.
(relay race)
'about to bet on' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to bet' becomes 'lay' (I've seen this in other clues).
'on' says to put letters next to each other.
're'+'lay'='RELAY'
(Other definitions for relay that I've seen before include "Team of workers" , "Broadcast" , "article in bank" , "supplementary store of anything" , "track event" .)