He may get no marks at all in maths! (6)

I believe the answer is:

thomas

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'he' is the definition.

I can't explain the remainder of the clue.

(Other definitions for thomas that I've seen before include "Writer who died in 1928" , "Ancient writing instruments" , "-- Moore, Irish poet" , "Doubting apostle" , "The Apostle who initially doubted the Resurrection" .)

I've seen this clue in the Evening Standard.

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