A dessert’s served up in layers (6)
I believe the answer is:
strata
'layers' is the definition.
(I know this)
'a dessert's served up' is the wordplay.
'dessert's' becomes 'tart's' (type of dessert).
'served up' says the letters should be written backwards (in a down clue, letters go up).
'a'+'tarts'='atarts'
'atarts' written backwards gives 'STRATA'.
'in' is the link.
(Other definitions for strata that I've seen before include "Tiers of thicknesses" , "Layers into which rocks are divided" , "Layers of rock on earth's surface" , "Layers as of rock in the earth's crust" , "Layers or social classes" .)