Nile viper (3)
I believe the answer is:
asp
'nile viper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for asp that I've seen before include "Slithery thing" , "It proved deadly for Cleopatra" , "snake appears" , "Cleopatra-killing snake" , "Cleopatra's poisoner" .)