Betting notes on rate (5)

I believe the answer is:

speed

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'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.

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(Other definitions for speed that I've seen before include "Film sensitivity to light" , "Alacrity" , "The going rate" , "Velocity" , "Travel quickly" .)

I've seen this clue in The Daily Express and The Mirror.

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