One studying another's books? (7)

I believe the answer is:

auditor

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'one studying another's books?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

This is the entire clue.

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(Other definitions for auditor that I've seen before include "Money expert" , "Accountant; hearer" , "Accounts checker" , "One certifying accounts" , "He checks the account books annually" .)

I've seen this clue in The Sun and The Times.

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