Unpopular mathematician (5)

I believe the answer is:

adder

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'unpopular mathematician' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

This is the entire clue.

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(Other definitions for adder that I've seen before include "Something you have to reckon with" , "exhibit in the reptile house" , "Venomous sort" , "1 of 21 [SNAKES]" , "An arithmetic snake" .)

I've seen this clue in The Globe and Mail.

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