Sieve mass of stones at bottom of mountain (6)

I believe the answer is:

screen

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'sieve' is the definition.
(synonyms)

I don't understand the rest of the clue.

(Other definitions for screen that I've seen before include "TV surface" , "Dividing partition" , "Investigate (someone) for suitability; conceal" , "'It provides protection, against insects, say (6)'" , "put on show" .)

I've seen this clue in The Daily Mail.

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