The most common and most likely answer to this crossword clue is the 10 letter word QUADRICEPS
I've seen this crossword clue in The Times
Q
U
A
D
R
I
C
E
P
S
Buddy explains!
The straight part
We know that "Muscle" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case muscle = quadriceps.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 QUADRICEPS mean?
QUADRICEPS noun- A muscle having four heads, especially the large extensor at the front of the thigh.