The difficulty in drug taking (3)
I believe the answer is:
fix
'the difficulty' is the definition.
(fix is a kind of difficulty)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for fix that I've seen before include "mess" , "Repair or attach" , "Predicament" , "Fasten" , "Addict's dose of drug" .)