Cleopatra's snake (3)
I believe the answer is:
'cleopatra's snake' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for asp that I've seen before include "Snake found in spa" , "Slithery thing" , "Joe Blake is Australian species" , "Snake, killed Cleopatra" , "Kind of snake - what a sap" .)