Drug turned into crack, mostly (8)
I believe the answer is:
'drug' is the definition.
(I know that drug is a type of drug)
'turned into crack mostly' is the wordplay.
'turned' indicates anagramming the letters.
'mostly' means to remove the last letter (most but not all of the word).
'intocrack' with its last letter removed is 'intocrac'.
'intocrac' with letters rearranged gives 'NARCOTIC'.
(Other definitions for narcotic that I've seen before include "Drug that induces sleep" , "Stupor-inducing drug" , "Opiate" , "drug supplied" , "Banned substance" .)