The most common and most likely answer to this crossword clue is the 9 letter word CREMASTER
C
R
E
M
A
S
T
E
R
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 = cremaster.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 CREMASTER mean?
CREMASTER noun- A thin muscle attached to the testicles.
- In the pupae of certain species of butterfly, a hook-shaped protuberance from the rear of the chrysalis casing, by which the caterpillar fixes itself to the pad of silk it has cemented to the underside of a perch.