Relaxed, nursing muscle that's valued (9)
I believe the answer is:
respected
'valued' is the definition.
('respect' can be a synonym of 'value')
'relaxed nursing muscle' is the wordplay.
'relaxed' becomes 'rested' (I've seen this before).
'nursing' indicates putting letters inside.
'muscle' becomes 'pec' (I've seen this in other clues).
'rested' enclosing 'pec' is 'RESPECTED'.
'that's' is the link.
(Other definitions for respected that I've seen before include "Looked up to" , "Regarded highly" , "Held in esteem" , "Had a high regard for someone" , "'Esteemed, showed deference to (9)'" .)