You're a loser, says European partner (9)
I believe the answer is:
checkmate
'you're a loser' is the definition.
'says european partner' is the wordplay.
'says' shows a homophone (sound like).
'european' becomes 'czech' (Czech is an example).
'partner' becomes 'mate' (synonyms).
'czech' is a homophone of 'check'.
'check'+'mate'='CHECKMATE'
(Other definitions for checkmate that I've seen before include "ending on board" , "defeat on board" , "Defeat totally" , "Chess game ending" , "The final killer move in chess" .)