The most common and most likely answer to this crossword clue is the 6 letter word FOUGHT
I've seen this crossword clue in The Daily Telegraph
F
O
U
G
H
T
Buddy explains!
The straight part
We know that "was involved in military action" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case was involved in military action = fought.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 FOUGHT mean?
FOUGHT verb- To contend in physical conflict, either singly or in war, battle etc.
- To contend in physical conflict with each other, either singly or in war, battle etc.
- To strive for something; to campaign or contend for success.
- To conduct or engage in (battle, warfare etc.).
- To engage in combat with; to oppose physically, to contest with.
- To try to overpower; to fiercely counteract.
- To cause to fight; to manage or manoeuvre in a fight.
- Of colours or other design elements: to clash; to fail to harmonize.