Running cadence of horse mostly circling donkey (4,4)

I believe the answer is:

step rate

'running cadence' is the definition.
(speed when running)

'horse mostly circling donkey' is the wordplay.
I cannot really understand how this works, but
'donkey' could be 'prat' (both can mean a foolish person) and 'prat' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'of' is the link.

I've seen this clue in The Guardian.

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