Sailor's muscles (3)
I believe the answer is:
abs
'muscles' is the definition.
(short for abdominals)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for abs that I've seen before include "Stomach muscles (abbr)" , "ratings?" , "Exercise to improve these" , "Some musculature" , "stopping system" .)