In a European it is courteous (6)
I believe the answer is:
polite
'courteous' is the definition.
(similar in meaning)
'in a european it' is the wordplay.
'in' is an insertion indicator.
'a european' becomes 'pole' (someone from Poland).
'pole' going around 'it' is 'POLITE'.
'is' is the link.
(Other definitions for polite that I've seen before include "Civil and good-mannered" , "Cultivated" , "Having good manners, socially correct" , "Minding one's p's and q's" , "Well-spoken and civil" .)