The most common and most likely answer to this crossword clue is the 8 letter word POST-WAR
I've seen this crossword clue in The Guardian
P
O
S
T
-
W
A
R
Buddy explains!
The straight part
We know that "when armistice really starts" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case when armistice really starts = post-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 POST-WAR mean?
POST-WAR adjective- Pertaining to a period of time immediately following the end of a war; where there is a cessation of conflict.