Beefiness (5)
I believe the answer is:
brawn
'beefiness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brawn that I've seen before include "Muscle; strength" , "Strength; a jellied loaf" , "This is strong" , "Beefy strength" , "Pressed cooked meat; strength" .)