The most common and most likely answer to this crossword clue is the 4 plus 2 plus 6 letter word HORS DE COMBAT
I've seen this crossword clue in The Times
H
O
R
S
D
E
C
O
M
B
A
T
More clues leading to the result HORS DE COMBAT
Buddy explains!
The straight part
We know that "out of action" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case out of action = hors de combat.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.