Legless reptile, mathematician basically? (5)

I believe the answer is:

adder

I'm a little stuck... Click here to teach me more about this clue!

'legless reptile' is the definition.
'adder' can be an answer for 'reptile' (adder is a kind of reptile). I'm not sure about the 'legless' bit.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for adder that I've seen before include "Reptilian" , "exhibit in the reptile house" , "Snake (like an abacus?)" , "1 of 21 [SNAKES]" , "Viper common in northern Eurasia" .)

I've seen this clue in The Telegraph.

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