Resistant setter's married a Parisienne! (6)
I believe the answer is:
immune
'resistant' is the definition.
(synonyms)
'setter's married a parisienne' is the wordplay.
'setter's' becomes 'im' (referring to the person who set the crossword).
'married' becomes 'm'.
'a parisienne' becomes 'une'.
'im'+'m'+'une'='IMMUNE'
(Other definitions for immune that I've seen before include "safe from a particular disease" , "Secure against, infection say" , "Safe from infection etc" , "Safe from attack" , "Cannot be infected" .)