Crossword Genius

River leaves long grotto (4)

Ross

I believe the answer is:

cave

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'grotto' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for cave that I've seen before include "Troglodyte's home" , "Hollow - warning" , "Natural subterranean opening" , "Subject of speleology" , "Warning - grotto" .)

I've seen this clue in The Mirror.
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