Crossword Genius

Mathematical crawler (5)

Ross

I believe the answer is:

adder

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'crawler' is the definition.
(snakes crawl)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for adder that I've seen before include "poisoner" , "One creature" , "Small poisonous snake, viper" , "a poisonous type" , "Venomous type" .)

I've seen this clue in The Mirror.
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