A venomous contributor (5)
I believe the answer is:
adder
'a venomous contributor' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
This is the entire clue.
Can you help me to learn more?
(Other definitions for adder that I've seen before include "One contributing more" , "Totter" , "Small poisonous snake, viper" , "1 of 21 [SNAKES]" , "Reptilian" .)