Bet against unprofessional song (3)
I believe the answer is:
lay
This clue is a triple definition!
'bet against' is the first definition.
(lay is a type of bet)
'unprofessional' is the second definition.
(as in a layman)
'song' is the third definition.
(lay is a kind of song)
(Other definitions for lay that I've seen before include "Narrative poem" , "like a minister? On the contrary" , "gamble" , "Unprofessional" , "Not clerical or professionally qualified" .)