Sure to upset employer (4)
I believe the answer is:
user
'employer' is the definition.
(someone who employs or uses something)
'sure to upset' is the wordplay.
'to upset' indicates anagramming the letters.
'sure' with letters rearranged gives 'USER'.
(Other definitions for user that I've seen before include "One who exploits, to be sure" , "One who works an appliance" , "Druggie" , "Operator; exploiter" , "one has to employ" .)