Fuss about an American soldier showing slow movement (6)
I believe the answer is:
adagio
'slow movement' is the definition.
(slow musical movement)
'fuss about an american soldier' is the wordplay.
'fuss' becomes 'ado' (synonyms).
'about' is an insertion indicator.
'an' becomes 'a'.
'american soldier' becomes 'gi'.
'a'+'gi'='agi'
'ado' placed around 'agi' is 'ADAGIO'.
'showing' is the link.
(Other definitions for adagio that I've seen before include "(Piece to be played) slowly" , "Leisurely tempo" , "develops slowly" , "moving slowly" , "(A movement) to be played slowly" .)