Employer in serious error (4)
I believe the answer is:
user
'employer' is the definition.
(someone who employs or uses something)
'in serious error' is the wordplay.
'in' says the answer is hidden in the clue.
'USER' is hidden in the letters of 'serious error'.
(Other definitions for user that I've seen before include "One exploiting others" , "Someone who exploits another" , "Utiliser" , "Operator; drug addict" , "Maybe one needing pot" .)