Treacherous one (5)
I believe the answer is:
viper
'treacherous one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for viper that I've seen before include "Venomous snake, person" , "treacherous type" , "A small poisonous snake" , "Adder" , "spiteful person" .)