Disable a horse with drugs (6)
I believe the answer is:
nobble
'disable a horse with drugs' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nobble that I've seen before include "Sabotage (racehorse, greyhound etc)" , "Seize (sl.)" , "Steal" , "Tamper with horse to prevent it winning" , "Dope (racehorse)" .)