A pastry's round layers (6)
I believe the answer is:
strata
'layers' is the definition.
(I know this)
'a pastry's round' is the wordplay.
'pastry's' becomes 'tart's' (type of pastry).
'round' is a reversal indicator.
'a'+'tarts'='atarts'
'atarts' back-to-front is 'STRATA'.
(Other definitions for strata that I've seen before include "Layers as of rock in the earth's crust" , "Layers of rock on earth's surface" , "Layers or thicknesses of earth" , "Layers into which rocks are divided" , "levels of society" .)