Addict starts to unload smack, entering rehab (4)
I believe the answer is:
user
'addict' is the definition.
(thesaurus)
'starts to unload smack entering rehab' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'unload smack entering rehab' is 'USER'.
(Other definitions for user that I've seen before include "Patron" , "Someone who exploits another" , "One exploiting others" , "Junkie's" , "Exploiter (of others)" .)