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 in reverse.
'after' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'nail' reversed gives '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" .)