A ruse to upset the employer (4)
I believe the answer is:
user
'the employer' is the definition.
(someone who employs or uses something)
'a ruse to upset' is the wordplay.
'to upset' indicates an anagram.
'ruse' anagrammed gives 'USER'.
(Other definitions for user that I've seen before include "Utiliser" , "Drug addict (informal)" , "one has to employ" , "One getting benefit from" , "Patron" .)