Polish capital ruined by lost war but it is in the past (3)
I believe the answer is:
was
'is in the past' 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 was that I've seen before include "(He/she/it) existed" , "Existed in the past" , "(I/he) existed" , "'Is, in the past (3)'" , "Lived" .)