Layers of stone on grass area (6)
I believe the answer is:
strata
'layers' is the definition.
(I know this)
'stone on grass area' is the wordplay.
'stone' becomes 'st'.
'on' says to put letters next to each other.
'grass' becomes 'rat' (both can mean a snitch or informer).
'area' becomes 'a' (maths abbreviation).
'st'+'rat'+'a'='STRATA'
'of' acts as a link.
(Other definitions for strata that I've seen before include "Layers or social classes" , "Levels, grades or social classes" , "bands" , "hard beds" , "Geological levels" .)