The most common and most likely answer to this crossword clue is the 7 letter word FETLOCK
I've seen this crossword clue in Independent
F
E
T
L
O
C
K
More clues leading to the result FETLOCK
- Originally, eight tendons in sheep joint
- Iron bolt secures West End joint
- Joint where gang takes in film
- Sheep eating last bits of the wiriest horse's hair
- Joint in ponies and, abroad, in many sheep
- Gather around extremely elegant bit of leg
- Vacuous eejit interrupting congregation, bit of a nag
- Alien surrounded by crowd in joint
- Crowd screening film in beastly joint
- Heat regularly seen in sheep or horse’s joint
Buddy explains!

The straight part
We know that "bit of a nag" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bit of a nag = fetlock.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does FETLOCK mean?
FETLOCK noun- A joint of the horse's leg below the knee or hock and above the hoof.
- The tuft of hair that grows at this joint.