Employer taken in because resentful (4)
I believe the answer is:
user
'employer' is the definition.
(someone who employs or uses something)
'taken in because resentful' is the wordplay.
'taken in' says the answer is hidden in the clue.
'USER' is hidden in the letters of 'because resentful'.
(Other definitions for user that I've seen before include "Drug taker" , "Consumer" , "Person who takes drugs" , "Manipulator" , "One taking drugs" .)