Soldier holds instrument with muscles (6)
I believe the answer is:
glutei
'muscles' is the definition.
(gluteus is a kind of muscle)
'soldier holds instrument' is the wordplay.
'soldier' becomes 'gi' (government issue or general issue).
'holds' means one lot of letters goes inside another.
'instrument' becomes 'lute' (lute is a kind of instrument).
'gi' going around 'lute' is 'GLUTEI'.
'with' acts as a link.
(Other definitions for glutei that I've seen before include "muscles" , "behind?" .)