Top horse to dominate (8)
I believe the answer is:
bestride
'dominate' is the definition.
(I've seen this in another clue)
'top horse' is the wordplay.
'top' becomes 'best' (associated in meaning).
'horse' becomes 'ride' (animal that can be ridden).
'best'+'ride'='BESTRIDE'
'to' acts as a link.
(Other definitions for bestride that I've seen before include "Straddle" , "Stand over protectively" , "Protect" , "get leg over?" , "Command" .)