Poisonous snakes, they reckon (6)
I believe the answer is:
adders
'poisonous snakes' is the definition.
(I've seen this in another clue)
'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 explanation may well be incorrect...
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" .)