Small smooth stone (6)
I believe the answer is:
pebble
'small smooth stone' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pebble that I've seen before include "'Small, rounded stone (6)'" , "There are many on a stony beach" , "Little rock" , "one of many on beach" , "Small fragment of rock" .)