Drug turned into crack, mostly (8)
I believe the answer is:
narcotic
'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).
'into'+'crack'='intocrack'
'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" .)