Loose mountainside stones (5)
I believe the answer is:
scree
'loose mountainside stones' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for scree that I've seen before include "Safeguard" , "Sloping mass of loose rock at the base of a cliff" , "Mass of loose rocks at the base of a cliff" , "Stony hillside" , "Steep, stony slope" .)