France’s very old-fashioned king, one invading others’ lands (10)
I believe the answer is:
trespasser
'one invading others' lands' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'france's very old-fashioned king' is the wordplay.
'france's very' becomes 'tres' (I can't explain this - if you can you should give a lot more credence to this answer).
'old-fashioned' becomes 'passe' (synonyms).
'king' becomes 'R' (abbreviation of Latin rex).
'tres'+'passe'+'r'='TRESPASSER'
Can you help me to learn more?
(Other definitions for trespasser that I've seen before include "Illegal enterer of land" , "One unlawfully entering another's land" , "One poaches, say" , "Palace intruder" , "Intruder into another's land" .)