Muscle condition (4)
I believe the answer is:
tone
'muscle condition' 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 "accent" , "Quality of sound, voice say" , "Note; note (anag.)" , "Quality or character of sound" , "Theobald Wolf . . . ." .)