Part of a horse's back (7)
I believe the answer is:
withers
'part of a horse's back' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for withers that I've seen before include "Bit of a nag" , "Languishes; part of horse near shoulder-blades" , "Ridge at the base of a horse'd neck, from which its height is measured" , "Writhes on ridge between a horse's shoulder-blades" , "Part between a horse's shoulder blades" .)