Capital happened to be unprepared in retreat (6)
I believe the answer is:
warsaw
'capital' is the definition.
(capital of Poland)
'happened to be unprepared in retreat' is the wordplay.
'happened to' becomes 'was' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'be unprepared' becomes 'raw' (I've seen this in another clue).
'in retreat' is a reversal indicator.
'was'+'raw'='wasraw'
'wasraw' backwards is 'WARSAW'.
Can you help me to learn more?
(Other definitions for warsaw that I've seen before include "Capital of a European Union state" , "Capital city on the Vistula" , "Polands capital" , "Capital city of Poland" , "Poland's largest city" .)