A pastrys rolled over in layers (6)
I believe the answer is:
strata
'layers' is the definition.
(I know this)
'a pastrys rolled over' is the wordplay.
'pastrys' becomes 'tarts' (tart is a kind of pastry).
'rolled over' is a reversal indicator.
'a'+'tarts'='atarts'
'atarts' back-to-front is 'STRATA'.
'in' is the link.
(Other definitions for strata that I've seen before include "Layers as of rocks in earth's crust" , "Geological layers" , "Layers into which rocks are divided" , "Levels, grades or social classes" , "Beds of sedimentary rock" .)