Well-developed muscles (5)
I believe the answer is:
brawn
'well-developed muscles' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for brawn that I've seen before include "Jellied meat -- not brain!" , "Strength; a jellied loaf" , "Pressed cooked meat; strength" , "Muscular strength" , "Muscle; strength" .)