The most common and most likely answer to this crossword clue is the 9 letter word PAINTBALL
I've seen this crossword clue in Financial Times
P
A
I
N
T
B
A
L
L
More clues leading to the result PAINTBALL
- Beer delivery outside a stag do activity?
- Taliban moving into place for quasi-military game
- Shooting game, agony unlikely without bullets, for a start
- It may be shot or longer drink, holding a party
- War re-enactment in which one gets decorated?
- War game is agony, and unlikely to include bishop
- Distress unlikely to involve British skirmishing where combatants get decorated
- Isn’t bishop wearing covering in this military game?
- Coloured cosmetics used before gathering for dancing game
- Drink bottles beginning to affect party game
Buddy explains!

The straight part
We know that "quasi-military game" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case quasi-military game = paintball.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 PAINTBALL mean?
PAINTBALL noun- A sport where teams shoot each other with gelatin capsules filled with paint-like dye.
- The dye-filled capsule fired in the sport of paintball.
- To engage in the sport of paintball.
- To shoot someone using a paintball gun.