Coming off horse, perhaps, in retreat (10)
I believe the answer is:
withdrawal
I believe this is a double definition.
'coming off horse perhaps' is the first definition.
(horse is slang for heroin)
'in retreat' is the second definition.
(I've seen this before)
(Other definitions for withdrawal that I've seen before include "Act of taking money out of an account" , "Retreat" , "Exit" , "Extraction" , "resignation" .)