They reckon they'll hide in the long grass (6)
I believe the answer is:
'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps there's a link between them I don't understand?
'reckon they'll hide in the long grass' is the wordplay.
I cannot quite understand how this works, but
'reckon' could be 'add' (reckon can mean to calculate) and 'add' is found 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 "at the zoo?" , "Mathematical snakes" , "Poisonous types" , "Do they deal with tots" , "Re Dad's snakes" .)