Betting three notes on rate (5)
I believe the answer is:
speed
'rate' is the definition.
(rate at which something is moving)
'betting three notes on' is the wordplay.
'three notes' means three substitutions for 'note'.
'betting' becomes 'sp'.
'note' becomes 'e' (musical note).
'note' becomes 'e' (musical note).
'note' becomes 'D' (musical note).
'on' is a charade indicator (letters next to each other).
'sp'+'e'+'e'+'d'='SPEED'
(Other definitions for speed that I've seen before include "Move quickly (and illegally?)" , "Travel quickly" , "'More haste, less . . . . .'" , "Alacrity" , "Rtae of progress" .)