The most common and most likely answer to this crossword clue is the 13 letter word NONCOMBATANTS
I've seen this crossword clue in Financial Times
N
O
N
C
O
M
B
A
T
A
N
T
S
Buddy explains!
The straight part
We know that "who will not fight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case who will not fight = noncombatants.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 NONCOMBATANTS mean?
NONCOMBATANTS noun- A non-fighting member of the armed forces.
- A civilian in time of conflict.