Get on highest roller coaster? (8)
I believe the answer is:
bestride
'get on' is the definition.
('bestride' can be a synonym of 'get on')
'highest roller coaster?' is the wordplay.
'highest' becomes 'best'.
'roller coaster?' becomes 'ride' (roller coaster is a kind of ride).
'best'+'ride'='BESTRIDE'
(Other definitions for bestride that I've seen before include "Dominate" , "span" , "Sit or stand over something imposingly" , "Stand over protectively" , "Do this to horse" .)