Pebble, rock (5)
I believe the answer is:
stone
I believe this is a double definition.
'pebble' is the first definition.
(I've seen this in another clue)
'rock' is the second definition.
(I know that rock can be written as stone)
(Other definitions for stone that I've seen before include "See 1" , "Rock; gem" , "Boulder" , "Hard lump of mineral matter" , "See 21" .)