Muscle in remaining eager (9)
I believe the answer is:
expectant
'eager' is the definition.
(thesaurus)
'muscle in remaining' is the wordplay.
'muscle' becomes 'pec' (I've seen this in other clues).
'in' indicates putting letters inside.
'remaining' becomes 'extant' (I've seen this before).
'pec' going inside 'extant' is 'EXPECTANT'.
(Other definitions for expectant that I've seen before include "Eagerly awaiting" , "Hopeful" , "in the club" , "Waiting for something" , "Pregnant" .)