Intruder very dated in French resistance (10)
I believe the answer is:
trespasser
'intruder' is the definition.
(synonyms)
'very dated in french resistance' is the wordplay.
'very' becomes 'tres' ('very' in French).
'dated in french resistance' becomes 'passer' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tres'+'passer'='TRESPASSER'
Can you help me to learn more?
(Other definitions for trespasser that I've seen before include "Palace intruder" , "one of a number praying for forgiveness?" , "Intruder into another's land" , "One unlawfully entering another's land" , "sinner" .)