Joint in ponies and, abroad, in many sheep (7)
I believe the answer is:
fetlock
'joint in ponies' is the definition.
'and abroad in many sheep' is the wordplay.
'and abroad' becomes 'et' ('and' in French).
'in' is an insertion indicator.
'many sheep' becomes 'flock'.
'et' going into 'flock' is 'FETLOCK'.
(Other definitions for fetlock that I've seen before include "Hair above horse's hoof" , "Equine leg joint" , "Part of horse's leg above and behind hoof" , "Horse's leg joint" , "Projection above a horse's hoof" .)