One maybe poaching very old hat from French king (10)
I believe the answer is:
trespasser
'one maybe poaching' is the definition.
(I know that poachers poach)
'very old hat from french king' is the wordplay.
'very' becomes 'tres' ('very' in French).
'old hat from french' becomes 'passe' ('passe' is a French word meaning outdated).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'tres'+'passe'+'r'='TRESPASSER'
(Other definitions for trespasser that I've seen before include "One poaches, say" , "Illegal enterer of land" , "sinner" , "Intruder into another's land" , "Misplaced individual" .)