Crossword Genius

Calculating snake (5)

Ross

I believe the answer is:

adder

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

'snake' is the definition.
(adder is a kind of snake)

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 "Viper common in northern Eurasia" , "Totting snake" , "Britain's only venomous snake" , "Venomous type" , "Something you have to reckon with" .)

I've seen this clue in The Irish Independent.
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