Endless mortar secures loose rocks (5)

I believe the answer is:

scree

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'loose rocks' is the definition.
(scree is a mass of loose rocks on a mountain)

I can't explain the rest of the clue.

(Other definitions for scree that I've seen before include "Stony hillside" , "Rock debris at the base of a cliff" , "loose stone" , "Mass of loose stones on mountainside" , "Safeguard" .)

I've seen this clue in The Telegraph.

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