Nile snake (3)
I believe the answer is:
asp
'nile snake' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for asp that I've seen before include "Snake, killed Cleopatra" , "Cleopatra's supposed nemesis" , "Reptile" , "Predatory fish" , "One secreting poison" .)