The most common and most likely answer to this crossword clue is the 7 letter word DILATOR
I've seen this crossword clue in The Times
D
I
L
A
T
O
R
More clues leading to the result DILATOR
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 = dilator.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 DILATOR mean?
DILATOR noun- Any nerve or muscle that causes part of the body to dilate
- Any drug that causes such dilation
- An instrument used to dilate an orifice or cavity