Mount most suitable horse, perhaps (8)
I believe the answer is:
bestride
'mount' is the definition.
(I know that mount can be written as bestride)
'most suitable horse perhaps' is the wordplay.
'most suitable' becomes 'best' (I've seen this in another clue).
'horse perhaps' becomes 'ride' (animal that can be ridden).
'best'+'ride'='BESTRIDE'
(Other definitions for bestride that I've seen before include "Protect" , "Sit or stand over something imposingly" , "Straddle" , "Dominate" , "get a legover?" .)