Drugs involved in underworld operations (4)
I believe the answer is:
dope
'drugs' is the definition.
(thesaurus)
'involved in underworld operations' is the wordplay.
'involved in' says the answer is hidden in the clue.
'DOPE' is hidden in the letters of 'underworld operations'.
(Other definitions for dope that I've seen before include "Fool - cannabis" , "Sucker" , "Weed perhaps" , "Information; nitwit" , "Charlie" .)