The most common and most likely answer to this crossword clue is the 7 letter word SLUMPED
I've seen this crossword clue in The Guardian
S
L
U
M
P
E
D
More clues leading to the result SLUMPED
Buddy explains!

The straight part
We know that "Fallen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fallen = slumped.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 SLUMPED mean?
SLUMPED verb- To collapse heavily or helplessly.
- To decline or fall off in activity or performance.
- To slouch or droop.
- To lump; to throw together messily.
- To fall or sink suddenly through or in, when walking on a surface, as on thawing snow or ice, a bog, etc.
- (slang) To cause to collapse; to hit hard; to render unsconscious; to kill.