He perhaps gets heroin cut (4)
I believe the answer is:
gash
'cut' is the definition.
(gash is a kind of cut)
'he perhaps gets heroin' is the wordplay.
'he perhaps' becomes 'gas' (he is a kind of gas).
'gets' is a charade indicator (letters next to each other).
'heroin' becomes 'h' (synonyms).
'gas'+'h'='GASH'
(Other definitions for gash that I've seen before include "Cut; waste (colloq.)" , "Cut deeply" , "injury" , "Ghastly" , "Jagged cut" .)