Because the pastry in it rises in layers (6)
I believe the answer is:
strata
'layers' is the definition.
(I know this)
'because the pastry in it rises' is the wordplay.
'because' becomes 'as' (I've seen this in other clues).
'the pastry' becomes 'tart' (type of pastry).
'in it' means one lot of letters goes inside another.
'rises' shows that the letters should be reversed in order.
'as' going around 'tart' is 'atarts'.
'atarts' backwards is 'STRATA'.
'in' acts as a link.
(Other definitions for strata that I've seen before include "Beds of sedimentary rock" , "Layers of sedimentary rock" , "Layers as of rocks in earth's crust" , "Layers or thicknesses of earth" , "Tiers of thicknesses" .)