Getting on top, on horseback (10)

I believe the answer is:

bestriding

'getting on' is the definition.
('bestride' can be a synonym of 'get on')

'top on horseback' is the wordplay.
'top' becomes 'best' (associated in meaning).
'on horseback' becomes 'riding' (I've seen this before).
'best'+'riding'='BESTRIDING'

(Another definition for bestriding that I've seen is " Dominating".)

I've seen this clue in The Sunday Times.

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