Betting notes on rate (5)
I believe the answer is:
speed
'rate' is the definition.
(rate at which something is moving)
'betting notes' is the wordplay.
'betting' becomes 'sp'.
'notes' becomes 'eed' (I can't justify this - if you can you should believe this answer much more).
'sp'+'eed'='SPEED'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for speed that I've seen before include "Film sensitivity to light" , "Alacrity" , "The going rate" , "Velocity" , "Travel quickly" .)