Muscle power (5)
'muscle power' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sinew that I've seen before include "fibre" , "power source" , "Tendon - ligament" , "source of physical strength" , "Gristle" .)
(Other definitions for brawn that I've seen before include "Pressed cooked meat; strength" , "perhaps, a sandwich filling" , "Beefiness" , "Jellied meat -- not brain!" , "Jellied meat - muscle" .)