Originally, eight tendons in sheep joint (7)
I believe the answer is:
fetlock
'joint' is the definition.
(fetlock is a kind of joint)
'originally eight tendons in sheep' is the wordplay.
'originally' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'sheep' becomes 'flock' (a group of sheep).
The initial letters of 'eight tendons' is 'et'.
'et' placed into 'flock' is 'FETLOCK'.
(Other definitions for fetlock that I've seen before include "Part of a charger" , "Horse's leg joint" , "Equine leg joint" , "Part of horse's leg above and behind hoof" , "beastly joint" .)