Old detective consumed son's drugs (7)
I believe the answer is:
opiates
'drugs' is the definition.
(opiate is a kind of drug)
'old detective consumed son's' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'detective' becomes 'PI' (private eye).
'consumed' becomes 'ate' (to eat is to consume food).
'o'+'pi'+'ate'+'s'='OPIATES'
(Other definitions for opiates that I've seen before include "Poppy-derived drugs" , "Is a poet (anag.)" , "Sedative drugs" , "Branch of narcotics" , "Narcotic drugs" .)