Operator in serious error (4)
I believe the answer is:
user
'operator' is the definition.
(I've seen this before)
'in serious error' is the wordplay.
'in' indicates the answer is hidden within the clue.
'USER' is hidden in the letters of 'serious error'.
(Other definitions for user that I've seen before include "With 1a [NEW] he'll need different login" , "Manipulator" , "One who exploits others" , "junkie" , "Person employing something" .)