The most common and most likely answer to this crossword clue is the 5 letter word RECTI
I've seen this crossword clue in Financial Times
R
E
C
T
I
More clues leading to the result RECTI
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 = recti.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 RECTI mean?
RECTI noun- Any of several straight muscles in various parts of the body, as of the abdomen, thigh, eye etc.