Hair on a horse's neck (4)
I believe the answer is:
mane
'hair on a horse's neck' 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 shaggy hair" , "Animal's neck hair" , "(Lion's) hair on neck" , "Long flowing hair" , "Hair (of lion)" .)