Vipers can make one sadder (6)
I believe the answer is:
adders
'vipers' is the definition.
(adder is a kind of viper)
'make one sadder' is the wordplay.
'make' is an anagram indicator.
'sadder' is an anagram of 'ADDERS'.
'can' acts as a link.
(Other definitions for adders that I've seen before include "Poisonous types" , "Small vipers" , "Do they deal with tots" , "Sadder with snakes" , "Mathematical snakes" .)