Small weight also included rock (9)
I believe the answer is:
sandstone
'rock' is the definition.
(sandstone is a kind of rock)
'small weight also included' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'weight' becomes 'stone' (stone is a unit of weight).
'also' becomes 'and' (both can mean in addition).
'included' indicates putting letters inside.
's'+'stone'='sstone'
'sstone' placed around 'and' is 'SANDSTONE'.
(Other definitions for sandstone that I've seen before include "Rock containing quartz grains" , "Type of sedimentary rock" , "Among the best of building materials" , "Such building material" .)