Employer is sure? Wrong! (4)
I believe the answer is:
user
'employer' is the definition.
(thesaurus)
'sure? wrong' is the wordplay.
'wrong' indicates anagramming the letters.
'sure' anagrammed gives 'USER'.
'is' acts as a link.
(Other definitions for user that I've seen before include "Right long-established" , "Patron" , "Maybe one needing pot" , "One who employs" , "Exploiter (of others)" .)