Employer admitted to serious error (4)

I believe the answer is:

user

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'employer' is the definition.
(someone who employs or uses something)

'admitted to serious error' is the wordplay.
'USER' is hidden in amongst the letters.
But, I'm not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for user that I've seen before include "With 1a [NEW] he'll need different login" , "Right long-established" , "Ruse for employer" , "Patron" , "Operator; drug addict" .)

I've seen this clue in The Sun.

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