Pebble or boulder (5)
I believe the answer is:
stone
This clue is a double definition.
'pebble' is the first definition.
(I've seen this in another clue)
'boulder' is the second definition.
(I've seen this in another clue)
(Other definitions for stone that I've seen before include "Olive pip" , "Piece of rock - weight" , "See 21" , "Imperial unit of weight" , "See 5" .)