Muscle; strength (5)
I believe the answer is:
brawn
This clue is a double definition.
'muscle' is the first definition.
(I know that muscle can be written as brawn)
'strength' is the second definition.
(I know that brawn is a type of strength)
(Other definitions for brawn that I've seen before include "This is strong" , "Beefiness" , "Pressed cooked meat; strength" , "Jellied meat -- not brain!" , "Strength; a jellied loaf" .)