Thus, for the Americans, he composed marches (5)
I believe the answer is:
sousa
'marches' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I don't see?
'thus for the americans he composed' is the wordplay.
'thus' becomes 'so' (synonyms).
'for' says to put letters next to each other.
'the americans' becomes 'US' (United States).
'he composed' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'so'+'us'+'a'='SOUSA'
Can you help me to learn more?
(Other definitions for sousa that I've seen before include "American composer known for marches" , "he produced the Washington Post" , "US composer of marches" , "John Philip --, American march composer" , "John Philip ..., composer of military marches" .)