A calculating creature? (5)

I believe the answer is:

adder

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'a calculating creature?' is the definition.
The definition and answer can be both animals as well as being singular nouns.
Maybe there's an association between them I don't understand?

This is all the clue.

Can you help me to learn more?

(Other definitions for adder that I've seen before include "Venomous type" , "Dread (anag)" , "1 of 21 [SNAKES]" , "Britain's only venomous snake" , "accountant?" .)

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