Suspicious cartons contain drug precursor (8)
I believe the answer is:
ancestor
'precursor' is the definition.
(thesaurus)
'suspicious cartons contain drug' is the wordplay.
'suspicious' is an anagram indicator (an unusual or suspect spelling).
'contain' indicates putting letters inside.
'drug' becomes 'e' (E is another name for ecstasy).
'cartons' anagrammed gives 'ancstor'.
'ancstor' enclosing 'e' is 'ANCESTOR'.
(Other definitions for ancestor that I've seen before include "descent comes from one" , "Predecessor" , "Enactors (anag)" , "Old member of the family -no traces (anag)" , "Adam was our first one" .)