Poisonous snakes, they reckon (6)
I believe the answer is:
adders
'poisonous snakes' is the definition.
(I've seen this before)
'they reckon' is the wordplay.
I cannot really understand how this works, but
'reckon' could be 'add' (reckon can mean to calculate) and 'add' is present in the answer.
The remaining letters 'ers' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for adders that I've seen before include "Sadder with snakes" , "Do they deal with tots" , "Venomous snakes" , "Mathematical snakes" , "Dreads (anag) -- snakes" .)