Their venom can make us sadder (6)
I believe the answer is:
adders
'their venom can' is the definition.
I can't judge whether this defines the answer.
'us sadder' is the wordplay.
'sadder' can be anagrammed to 'ADDERS'.
Though, I am not sure how the anagram is indicated.
'make' acts as a link.
Can you help me to learn more?
(Other definitions for adders that I've seen before include "Poisonous snakes" , "Small vipers" , "Dreads (anag) -- snakes" , "Mathematical snakes" , "Arithmetic snakes?" .)