A pastry's turned into layers (6)
I believe the answer is:
strata
'layers' is the definition.
(stratum is a kind of layer)
'a pastry's turned' is the wordplay.
'pastry's' becomes 'tarts' (tart is a kind of pastry).
'turned' says the letters should be written backwards.
'a'+'tarts'='atarts'
'atarts' back-to-front is 'STRATA'.
'into' is the link.
(Other definitions for strata that I've seen before include "Layers as of rocks in earth's crust" , "Layers into which rocks are divided" , "Layers as of rock in the earth's crust" , "Layers of rock on earth's surface" , "Parallel layers" .)