The mad solution with single drug (9)
I believe the answer is:
methadone
'drug' is the definition.
(methadone is a kind of drug)
'the mad solution with single' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'the' is 'eth' which is located in the answer.
'single' could be 'one' (a single thing - only one) and 'one' is located in the answer.
The remaining letters 'mad' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for methadone that I've seen before include "Drug used in treating heroin addiction" , "Synthetic drug taken to replace heroin" , "Narcotic analgesic drug" , "Synthetic analgesic" .)