French resort invaded by English relation (5)
I believe the answer is:
niece
'relation' is the definition.
(I know that niece is a type of kinswoman)
'french resort invaded by english' is the wordplay.
'french resort' becomes 'nice' (French coastal resort).
'invaded by' means one lot of letters goes inside another.
'english' becomes 'E' (abbreviation).
'nice' placed around 'e' is 'NIECE'.
(Other definitions for niece that I've seen before include "Family relative" , "Female relation" , "Brother's girl" , "A younger female relative" , "member of different generation" .)