It's sure to upset an employer (4)
I believe the answer is:
user
'an employer' is the definition.
(thesaurus)
'it's sure to upset' is the wordplay.
'to upset' is an anagram indicator.
'sure' anagrammed gives 'USER'.
(Other definitions for user that I've seen before include "Consumer" , "Operator; drug addict" , "One who works an appliance" , "Substance taker" , "Someone who exploits another" .)