Addict rues mistake (4)
I believe the answer is:
user
'addict' is the definition.
(I've seen this in another clue)
'rues mistake' is the wordplay.
'mistake' indicates an anagram.
'rues' is an anagram of 'USER'.
(Other definitions for user that I've seen before include "Patron" , "Drug addict (informal)" , "Drug addict (colloq.)" , "Ruse for employer" , "One who exploits others" .)