A tarts upset at the layers (6)
I believe the answer is:
strata
'the layers' is the definition.
(stratum is a kind of layer)
'a tarts upset' is the wordplay.
'upset' says the letters should be written backwards.
'a'+'tarts'='atarts'
'atarts' back-to-front is 'STRATA'.
'at' acts as a link.
(Other definitions for strata that I've seen before include "Layers as of rocks in earth's crust" , "Layers as of rock in the earth's crust" , "Layers of sedimentary rock" , "Layers of rock on earth's surface" , "Layers or thicknesses of earth" .)