The French duly concluded he could paint (4)
I believe the answer is:
lely
'paint' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?
'the french duly concluded he' is the wordplay.
'the french' becomes 'le' ('the' in French).
'duly concluded he' becomes 'ly' (I can't justify this - if you can you should believe this answer much more).
'le'+'ly'='LELY'
'could' is the link.
Can you help me to learn more?
(Other definitions for lely that I've seen before include "Dutch painter" , "Peter -, C 17 portraitist" , "Dutch artist painting in England" , "Sir Peter -, C 17 painter" .)