Addict's escort has no heroin (4)
I believe the answer is:
user
'addict's' is the definition.
(I've seen this before)
'escort has no heroin' is the wordplay.
'escort' becomes 'usher' (usher is a kind of escort).
'has no' suggests deleting specific letters.
'heroin' becomes 'h' (synonyms).
'usher' with 'h' taken away is 'USER'.
(Other definitions for user that I've seen before include "Manipulative woman" , "One taking advantage" , "Junkie's" , "Right long-established" , "One exploiting others" .)