Keep last of muscle and strength (5)
I believe the answer is:
forte
'strength' is the definition.
(synonyms)
'keep last of muscle' is the wordplay.
'keep' becomes 'fort' (I've seen this in other clues).
'last of' indicates one should take the final letters.
The final letter of 'muscle' is 'e'.
'fort'+'e'='FORTE'
'and' is the link.
(Other definitions for forte that I've seen before include "speciality" , "Skill" , "Loud (mus.)" , "Personal strong suit" , "Loud (music)" .)