Smooth beach surface (4)
I believe the answer is:
sand
This might be a double definition.
'smooth' is the first definition.
(I know that sand is a more specific form of smooth)
'beach surface' is the second definition.
'sand' can be an answer for 'beach'. I'm unsure of the 'surface' bit.
Can you help me to learn more?
(Other definitions for sand that I've seen before include "yellowish-brown" , "Egg timer grains" , "Rock particles; smooth" , "Loose grains" , "Result of rock erosion" .)