10 across
Rocks
falling
and
getting
into
dell
is
ruinous
(9)
Rocks falling and getting into dell is ruinous (9)
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The definition is at the beginning of the clue.
How the clue works
The definition here is Rocks falling, which is what a LANDSLIDE is. Take the word "ruinous" as the signal to jumble up the letters of "dell is", then drop AND (from "and" in the surface) into the middle of that anagram to build the whole thing.