Betting notes at a rate (5)
I believe the answer is:
speed
'rate' is the definition.
(rate at which something is moving)
'betting notes at a' is the wordplay.
'betting' becomes 'sp'.
'notes' becomes 'ee' (two E's - as in the musical note E).
'at a' becomes 'd' (I can't justify this - if you can you should give a lot more credence to this answer).
'sp'+'ee'+'d'='SPEED'
Can you help me to learn more?
(Other definitions for speed that I've seen before include "Alacrity" , "'More haste, less . . . . .'" , "Hasten" , "The going rate" , "Expedition" .)