One with leading part in drug production (6)
I believe the answer is:
heroin
'drug production' is the definition.
'heroin' can be an answer for 'drug' (I've seen this before). I am not sure about the 'production' bit.
'one with leading part' is the wordplay.
'one with leading' becomes 'hero' (I can't justify this - if you can you should give a lot more credence to this answer).
'part' becomes 'in part' (synonyms).
'hero'+'in'='HEROIN'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for heroin that I've seen before include "Narcotic powder derived from morphine" , "Hard drug" , "Highly addictive illegal drug" , "Highly addictive narcotic" , "junk" .)