Nancy's very old-fashioned royal interloper (10)
I believe the answer is:
trespasser
'interloper' is the definition.
(I know that interloper can be written as trespasser)
'nancy's very old-fashioned royal' is the wordplay.
'nancy's very' becomes 'tres' ('very' in French - Nancy is a French city).
'old-fashioned' becomes 'passe' (synonyms).
'royal' becomes 'r' (common abbreviation e.g. RM - Royal Marine).
'tres'+'passe'+'r'='TRESPASSER'
(Other definitions for trespasser that I've seen before include "one of a number praying for forgiveness?" , "Palace intruder" , "One entering land without permission" , "One unlawfully entering another's land" , "One poaches, say" .)