Rise in level of horse riding (5,2)

I believe the answer is:

mount up

'rise in level' is the definition.

'horse riding' is the wordplay.
'horse' becomes 'mount' (mount is a kind of horse**).
'riding' becomes 'up' (I've seen this in other clues).
'mount'+'up'='MOUNT UP'

'of' acts as a link.

(Another definition for mount up that I've seen is " Accumulate".)

I've seen this clue in The Times.

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