Snakes -- or their headless companions? (6)
I believe the answer is:
adders
'snakes' is the definition.
(adder is a kind of snake)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for adders that I've seen before include "Mathematical snakes" , "Totting snakes" , "Mathematical reptiles" , "Poisonous snakes" , "Totting 25 down?" .)