A topless pie’s rising layers (6)
I believe the answer is:
strata
'layers' is the definition.
(I know this)
'a topless pie's rising' is the wordplay.
'topless pie's' becomes 'tarts' (I can't justify this - if you can you should believe this answer much more).
'rising' says the letters should be written in reverse (in a down clue, letters go up).
'a'+'tarts'='atarts'
'atarts' backwards is 'STRATA'.
Can you help me to learn more?
(Other definitions for strata that I've seen before include "Layers of rock on earth's surface" , "Layers as of rocks in earth's crust" , "Geological levels" , "bands" , "Geological layers" .)