Crossword Genius

They are cold-blooded and calculating! (6)

Ross

I believe the answer is:

adders

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

'they' is the definition.
Both the definition and answer are plural nouns.
Maybe they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for adders that I've seen before include "Poisonous types" , "Snakes are totters" , "Poisonous snakes" , "Sadder with snakes" , "Mathematical reptiles" .)

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