They are cold-blooded and calculating! (6)
I believe the answer is:
adders
'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" .)