Collar reversed after one thanks European (7)
I believe the answer is:
italian
'european' is the definition.
(I know that Italian is a type of european)
'collar reversed after one thanks' is the wordplay.
'collar' becomes 'nail' (synonyms).
'reversed' says the letters should be written backwards.
'after' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'nail' back-to-front is 'lian'.
'i'+'ta'='ita'
'lian' after 'ita' is 'ITALIAN'.
(Other definitions for italian that I've seen before include "Nationality of benito or Giuseppe" , "Foreigner" , "(Meal in a) pizzeria" , "This European is anti Lia, oddly" , "A Latin I know from Rome, say" .)