Straddle top mount? (8)
I believe the answer is:
bestride
'straddle' is the definition.
(I've seen this before)
'top mount?' is the wordplay.
'top' becomes 'best' (associated in meaning).
'mount?' becomes 'ride' ('ride' can be a synonym of 'mount').
'best'+'ride'='BESTRIDE'
(Other definitions for bestride that I've seen before include "span" , "get leg over?" , "Sit or stand over something imposingly" , "Mounted" , "Have a leg either side of" .)