Concur with a European, mostly (5)
I believe the answer is:
agree
'concur with' is the definition.
(I've seen this in another clue)
'a european mostly' is the wordplay.
'european' becomes 'greek' (Greece is in Europe).
'mostly' means to remove the last letter (most but not all of the word).
'greek' with its final letter taken away is 'gree'.
'a'+'gree'='AGREE'
(Other definitions for agree that I've seen before include "Allow" , "Say yes to a suggestion" , "Concur, coincide" , "Accord, match" , "Match; tally" .)