Old heads taking current drugs (7)
I believe the answer is:
opiates
'drugs' is the definition.
(opiate is a kind of drug)
'old heads taking current' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'heads' becomes 'pates'.
'taking' means one lot of letters goes inside another.
'current' becomes 'i' (symbol for electric current).
'pates' placed around 'i' is 'piates'.
'o'+'piates'='OPIATES'
(Other definitions for opiates that I've seen before include "Branch of narcotics" , "Is a poet (anag.)" , "Poppy-derived drugs" , "Makes dull" , "Sedative drugs" .)