Concerning bets on races (6)
I believe the answer is:
relays
'races' is the definition.
(relay races)
'concerning bets on' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'bets' becomes 'lays' (I've seen this in another clue).
'on' means one lot of letters go next to another.
're'+'lays'='RELAYS'
(Other definitions for relays that I've seen before include "Races with teams and batons" , "Broadcasts" , "Foot races in which batons are passed" , "Passes on in multi-runner races" , "Transmits" .)