Female soldier with old piece of music (6)
I believe the answer is:
adagio
'piece of music' is the definition.
(adagio is a kind of piece of music)
'female soldier with old' is the wordplay.
'female' becomes 'Ada' (woman's name).
'soldier' becomes 'GI' (term for US soldier).
'with' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'ada'+'gi'+'o'='ADAGIO'
(Other definitions for adagio that I've seen before include "going slowly in work" , "Slowly and graceful, in music" , "leisurely movement" , "Slow musical passage" , "Movement in slow time" .)