The most common and most likely answer to this crossword clue is the 6 letter word RAINED
R
A
I
N
E
D
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The straight part
We know that "fell down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fell down = rained.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 RAINED mean?
RAINED verb- To have rain fall from the sky.
- To fall as or like rain.
- To issue (something) in large quantities.
- To reign.