The most common and most likely answer to this crossword clue is the 7 letter word PRE-WAR
I've seen this crossword clue in The Guardian
P
R
E
-
W
A
R
More clues leading to the result PRE-WAR
Buddy explains!

The straight part
We know that "before September 1939" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case before september 1939 = pre-war.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PRE-WAR mean?
PRE-WAR adjective- Before a war.