A flan's flipping layers (6)
I believe the answer is:
strata
'layers' is the definition.
(I know this)
'a flan's flipping' is the wordplay.
'flan's' becomes 'tart's'.
'flipping' shows that the letters should be reversed in order.
'a'+'tarts'='atarts'
'atarts' in reverse letter order is 'STRATA'.
(Other definitions for strata that I've seen before include "Layers as of rocks in earth's crust" , "Levels, grades or social classes" , "Layers or social classes" , "Layers as of rock in the earth's crust" , "hard beds" .)