"Keep Calm and Carry On", say, in capital city! (6)
I believe the answer is:
warsaw
'capital city' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for warsaw that I've seen before include "Capital city of Poland" , "1929 international agreement regulating commercial air transport" , "Polands capital" , "Poland's largest city" , "Capital city on the Vistula" .)