Fuss about a soldier that's played slowly (6)
I believe the answer is:
adagio
'that's played slowly' is the definition.
(musical term)
'fuss about a soldier' is the wordplay.
'fuss' becomes 'ado' (synonyms).
'about' is an insertion indicator.
'soldier' becomes 'GI' (term for US soldier).
'a'+'gi'='agi'
'ado' placed around 'agi' is 'ADAGIO'.
(Other definitions for adagio that I've seen before include "Musical composition to be played slowly and gracefully" , "Musically, in slow time" , "' - - for Strings' (Barber)" , "In music, leisurely" , "'Slowly and gracefully, in music (6)'" .)