Regard repose as protection for muscle (7)
I believe the answer is:
respect
'regard' is the definition.
(respecting is a kind of regarding)
'repose as protection for muscle' is the wordplay.
'repose' becomes 'rest' ('rest' can be a synonym of 'repose').
'as protection for' indicates putting letters inside.
'muscle' becomes 'pec' (I've seen this in other clues).
'rest' enclosing 'pec' is 'RESPECT'.
(Other definitions for respect that I've seen before include "Deference, obedience" , "Feeling of deep admiration" , "value" , "Pacific republic" , "Show honour to" .)