Rounded stone (6)
I believe the answer is:
pebble
'rounded stone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pebble that I've seen before include "There are many on a stony beach" , "Smooth little stone" , "Small, round stone" , "'Small, rounded stone (6)'" , "Small roundish stone" .)