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