Addict starts to undergo self-elected rehabilitation (4)
I believe the answer is:
user
'addict' is the definition.
(thesaurus)
'starts to undergo self-elected rehabilitation' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'undergo self elected rehabilitation' is 'USER'.
(Other definitions for user that I've seen before include "Junkie's" , "Ruse (anag.)" , "One who works an appliance" , "Exploiter (of others)" , "Operator; exploiter" .)