Night stalker (3)
I believe the answer is:
owl
'night stalker' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for owl that I've seen before include "Someone solemn or wise or staying up at night" , "Bird of the 18" , "paragon of wisdom?" , "Wise figure" , "Head-swivelling night bird" .)