Vipers (6)
I believe the answer is:
adders
'vipers' is the definition.
(adder is a kind of viper)
This is the entire clue.
(Other definitions for adders that I've seen before include "Do they deal with tots" , "sum? We do" , "Snakes are totters" , "Venomous snakes" , "Totting 25 down?" .)