Addict given bum steer off and on (4)
I believe the answer is:
user
'addict' is the definition.
(drug user)
'bum steer off and on' is the wordplay.
'off and on' indicates alternate letters (letters used at regular intervals).
'bum'+'steer'='bumsteer'
The alternating letters of 'bumsteer' are 'USER'.
'given' acts as a link.
(Other definitions for user that I've seen before include "Drug addict (informal)" , "Tech support caller?" , "Consumer" , "Patron" , "Right long-established" .)