US soldier squeezes instrument to show muscles (6)
I believe the answer is:
glutei
'muscles' is the definition.
(gluteus is a kind of muscle)
'us soldier squeezes instrument' is the wordplay.
'us soldier' becomes 'gi' (I've seen this in other clues).
'squeezes' means one lot of letters goes inside another (squeeze can mean hug or embrace).
'instrument' becomes 'lute' (lute is a kind of instrument).
'gi' placed around 'lute' is 'GLUTEI'.
'to show' acts as a link.
(Other definitions for glutei that I've seen before include "muscles" , "behind?" .)