Nile serpent (3)
I believe the answer is:
asp
'nile serpent' 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 "creeper" , "A glider's what's called for" , "Reptile" , "Snake seen in a spa, strangely" , "Joe Blake is Australian species" .)