A powerful narcotic (5)
I believe the answer is:
opium
'a powerful narcotic' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for opium that I've seen before include "De Quincey confessed to eating it" , "Addictive narcotic" , "wars with China" , "Narcotic derived from poppies" , "powerful drug" .)