Layers of rock (6)
I believe the answer is:
strata
'layers of rock' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for strata that I've seen before include "hard beds" , "Layers as of rock in the earth's crust" , "Layers of rock on earth's surface" , "Beds of sedimentary rock" , "Layers as of rocks in earth's crust" .)