Employer detected in Calculus error (4)

I believe the answer is:

user

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

'in calculus error' is the wordplay.
'in' says the answer is hidden in the clue.
'USER' is hidden in the letters of 'calculus error'.

'detected' acts as a link.
This may not be correct. It may belong to another bit of the clue.

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(Other definitions for user that I've seen before include "Drug addict (informal)" , "Operator; drug addict" , "Maybe one needing pot" , "Drug-taker" , "Ruse for employer" .)

I've seen this clue in The Sun.

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