Furtive one (5)
I believe the answer is:
sneak
'furtive one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sneak that I've seen before include "Move furtively - informer" , "In school, tell tales" , "To go stealthily" , "Snitcher" , "peach" .)