Horsehair (4)
I believe the answer is:
mane
'horsehair' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mane that I've seen before include "Long flowing hair" , "Lion or horse's 10 Across" , "Long shaggy hair" , "Neck hair of an animal" , "Animal's neck hair" .)