The most common and most likely answer to this crossword clue is the 4 plus 10 letter word LOST GENERATION
I've seen this crossword clue in Independent
L
O
S
T
G
E
N
E
R
A
T
I
O
N
More clues leading to the result LOST GENERATION
Buddy explains!

The straight part
We know that "fallen WWI soldiers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fallen wwi soldiers = lost generation.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.