A poet, I fancy narcotic (6)
I believe the answer is:
opiate
'narcotic' is the definition.
(opiate is a kind of narcotic)
'a poet i fancy' is the wordplay.
'fancy' indicates anagramming the letters.
'a'+'poet'+'i'='apoeti'
'apoeti' anagrammed gives 'OPIATE'.
(Other definitions for opiate that I've seen before include "drug to aid rest" , "Drug from poppies" , "painkiller" , "Drug made from poppy" , "of heroin, perhaps" .)