Adder (5)
I believe the answer is:
viper
'adder' is the definition.
(I know that adder can be written as viper)
This is all the clue.
(Other definitions for viper that I've seen before include "poisonous type" , "Malicious person" , "Adder" , "Malicious type" , "A poisonous snake or treacherous person" .)