Muscle firmness (4)
I believe the answer is:
tone
'muscle firmness' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tone that I've seen before include "Note as in voice" , "Quality of a person's voice" , "Bodily firmness" , "Vocal sound of note" , "Quality of a musical or vocal sound" .)