Old detective consumed narcotic (6)
I believe the answer is:
opiate
'narcotic' is the definition.
(opiate is a kind of narcotic)
'old detective consumed' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'detective' becomes 'pi' (I've seen this in other clues).
'consumed' becomes 'ate' (eating is a kind of consuming).
'o'+'pi'+'ate'='OPIATE'
(Other definitions for opiate that I've seen before include "dope" , "Sedating drug" , "Poppy-derived drug" , "Drug inducing a false sense of contentment" , "description of religion?" .)