Large masses of rock (8)
I believe the answer is:
boulders
'large masses of rock' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for boulders that I've seen before include "Huge 17s" , "Large smooth rocks" , "Large rounded rocks" , "Large pieces of rock worn smooth by weathering" , "Rocks larger than cobbles" .)