Very unfashionable French redhead is an intruder (10)
I believe the answer is:
trespasser
'an intruder' is the definition.
('trespasser' can be a synonym of 'intruder')
'very unfashionable french redhead' is the wordplay.
'very unfashionable french' becomes 'trespasse' ('very unfashionable' in French).
'redhead' becomes 'r' (the head/first letter of 'red').
'trespasse'+'r'='TRESPASSER'
'is' acts as a link.
(Other definitions for trespasser that I've seen before include "One entering land without permission" , "sinner" , "one of a number praying for forgiveness?" , "Intruder into another's land" , "Illegal enterer of land" .)