Mount finest horse (8)
I believe the answer is:
bestride
'mount' is the definition.
('bestride' can be a synonym of 'mount')
'finest horse' is the wordplay.
'finest' becomes 'best' ('best' can be similar in meaning to 'finest').
'horse' becomes 'ride'.
'best'+'ride'='BESTRIDE'
(Other definitions for bestride that I've seen before include "Get across" , "Command" , "Mounted" , "span" , "Stand over protectively" .)