Was in possession of drugs for underworld (5)
I believe the answer is:
hades
'for underworld' is the definition.
(Hades can mean the underworld in Greek myth)
'was in possession of drugs' is the wordplay.
'was in possession of' becomes 'had' (I've seen this in another clue).
'drugs' becomes 'es' (more than one E or Ecstasy tablet).
'had'+'es'='HADES'
(Other definitions for hades that I've seen before include "Classic hell in the shade" , "Inferno" , "Greek underworld with spirits of the dead" , "Underworld, hell" , "In Greek myth, the Underworld" .)