Illegal drug; idiot (4)
I believe the answer is:
dope
This clue is a double definition.
'illegal drug' is the first definition.
(I've seen this before)
'idiot' is the second definition.
(I've seen this in another clue)
(Other definitions for dope that I've seen before include "Fool - model aeroplane varnish" , "Dolt -- drug" , "Weed perhaps" , "Drug - information" , "Inside information (slang)" .)