Druggie shooting up in Baltimore subway (4)
I believe the answer is:
user
'druggie' is the definition.
(I've seen this before)
'shooting up in baltimore subway' is the wordplay.
'shooting up' says the letters should be written in reverse.
'in' indicates a hidden word.
'resu' is hidden in the letters of 'baltimore subway'.
'resu' reversed gives 'USER'.
(Other definitions for user that I've seen before include "Operator; drug addict" , "Person taking drugs" , "One getting benefit from" , "Patron" , "One exploiting others" .)