Get on top mount (8)
I believe the answer is:
bestride
'get on' is the definition.
('bestride' can be a synonym of 'get on')
'top mount' is the wordplay.
'top' becomes 'best' ('best' can be similar in meaning to 'top'**).
'mount' becomes 'ride' (riding is a kind of mounting).
'best'+'ride'='BESTRIDE'
(Other definitions for bestride that I've seen before include "Have a leg either side of" , "get a legover?" , "span" , "Get across" , "Mounted" .)