Crossword Genius

Low, venomous, calculating creatures? (6)

Ross

I believe the answer is:

adders

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'low venomous calculating creatures?' is the definition.
The answer and definition can be both animals as well as being plural nouns.
Perhaps there's a link between them I don't understand?

This is the entire clue.

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(Other definitions for adders that I've seen before include "Poisonous types" , "People doing sums" , "Snakes are totters" , "Re Dad's snakes" , "Arithmetic snakes?" .)

I've seen this clue in the Evening Standard.
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