Muscle or musical quality (4)
I believe the answer is:
tone
I believe this is a double definition.
'muscle' is the first definition.
(thesaurus)
'musical quality' is the second definition.
(I've seen this in another clue)
(Other definitions for tone that I've seen before include "Colour quality; note" , "Note; note (anag.)" , "Note as in voice" , "feel" , "Pitch (sound)" .)