The most common and most likely answer to this crossword clue is the 5 letter word QUADS
I've seen this crossword clue in The Guardian
Q
U
A
D
S
Buddy explains!
The straight part
We know that "Muscles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case muscles = quads.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 QUADS mean?
QUADS noun- Four shots of espresso.
- A kind of round-robin tournament between four players, where each participant plays every other participant once.
- The Bible, Book of Mormon, Doctrine and Covenants, and Pearl of Great Price bound in a single volume.
- A poster, measuring forty by thirty inches, advertising a cinematic film release.
- A quadrangle (courtyard).
- A quadruplet (infant).
- A quadrilateral.
- The quadriceps muscle.
- A quadriplegic person.
- A quadrupel beer.
- A quadcopter.
- A quad bike (from quadricycle)
- Abbreviation of quadrillion BTU. (1015 BTU)
- A blank metal block used to fill short lines of type.
- A joke used to fill long days of setting type.
- (phototypesetting and digital typesetting) A keyboard command which aligns text with the left or right margin, or centred between them. In combination, as quad left, quad right or quad centre.
- (exercise) quadriceps (thigh muscles)
- Four of a kind