Beach pebble or rock (9)
I believe the answer is:
sandstone
'rock' is the definition.
(sandstone is a kind of rock)
'beach pebble' is the wordplay.
'beach' becomes 'sand'.
'pebble' becomes 'stone' (I've seen this before).
'sand'+'stone'='SANDSTONE'
'or' acts as a link.
(Other definitions for sandstone that I've seen before include "Such building material" , "Type of sedimentary rock" , "Rock containing quartz grains" , "Among the best of building materials" .)