Ride this ultimately — and go off heroin? (6,5)
I believe the answer is:
saddle horse
'ride' is the definition.
'this ultimately and go off heroin?' is the wordplay.
'ultimately' indicates one should take the final letters.
'and' is a charade indicator (letters next to each other).
'go off' becomes 'addle' (eg an egg might addle).
'heroin?' becomes 'horse' ('horse' can be a synonym of 'heroin').
The last letter of 'this' is 's'.
's'+'addle'+'horse'='SADDLE-HORSE'
(Other definitions for saddle horse that I've seen before include "Mount" , "Wooden frame used to clean and store riding tackle" .)