Pieces in article welcome singular treaties (10)
I believe the answer is:
agreements
'treaties' is the definition.
(I know this)
'pieces in article welcome singular' is the wordplay.
'pieces' becomes 'men' (man can mean a chess piece).
'in' means one lot of letters goes inside another.
'article' becomes 'a' (the indefinite article in English).
'welcome singular' becomes 'greets' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'greets'='agreets'
'men' inserted within 'agreets' is 'AGREEMENTS'.
Can you help me to learn more?
(Other definitions for agreements that I've seen before include "contracts" , "Concerts" , "pledges" , "Compromises in" .)