Capital of Poland (6)
I believe the answer is:
warsaw
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">capital of poland</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for warsaw that I've seen before include "European city (with a Convention for commercial flying)" , "Largest city in Poland" , "Polish capital" , "1929 international agreement regulating commercial air transport" , "Poland's largest city" .)