An old barbarian acquired heroin (4)
I believe the answer is:
goth
'an old barbarian' is the definition.
'Goth' can be an answer for 'barbarian' (synonyms). I'm not sure about the 'an old' bit.
'acquired heroin' is the wordplay.
'acquired' becomes 'got' ('get' can be a synonym of 'acquire').
'heroin' becomes 'H' (H is a slang term for heroin).
'got'+'h'='GOTH'
Can you help me to learn more?
(Other definitions for goth that I've seen before include "One of Germanic tribe that invaded Roman empire" , "Old German; rock style" , "One of an ancient Germanic nation" , "Youngster clad in black" , "Uncivilised type" .)