A mass of small stones (6)
I believe the answer is:
gravel
'a mass of small stones' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for gravel that I've seen before include "Rock fragments and pebbles" , "bits of rock" , "Stone mixture for paths" , "Coarse sand mixed with small stones" , "Loose fragments of rock" .)