Viper feature (4)
I believe the answer is:
fang
'viper feature' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for fang that I've seen before include "Large pointed tooth" , "Venomous tooth" , "Snake tooth" , "Eg, wolf or snake's tooth" , "'Large, sharp tooth (4)'" .)