Employer is unusually sure (4)
I believe the answer is:
user
'employer' is the definition.
(thesaurus)
'unusually sure' is the wordplay.
'unusually' indicates an anagram.
'sure' anagrammed gives 'USER'.
'is' is the link.
(Other definitions for user that I've seen before include "Operator; exploiter" , "Druggie" , "One operating something" , "Drug addict (colloq.)" , "Consumer" .)