Layers of pastry in a small turnover (6)
I believe the answer is:
strata
'layers' is the definition.
(I know this)
'pastry in a small turnover' is the wordplay.
'pastry' becomes 'tart' (type of pastry).
'in' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'turnover' shows that the letters should be reversed in order (the letters turn over).
'a'+'s'='as'
'tart' inserted into 'as' is 'atarts'.
'atarts' written backwards gives 'STRATA'.
'of' acts as a link.
(Other definitions for strata that I've seen before include "Layers as of rocks in earth's crust" , "Geological levels" , "Levels, grades or social classes" , "Layers into which rocks are divided" , "Layers of rock on earth's surface" .)