Crossword Genius

Endlessly questioned lunar depression (5)

Ross

I believe the answer is:

rille

I'm a little stuck... Click here to teach me more about this clue!

'lunar depression' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for rille that I've seen before include "satellite channel" , "Channel on the moon" , "Fissure on the moon" .)

I've seen this clue in the Sydney Morning Herald.
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