Classes in arts return thanks (6)
I believe the answer is:
strata
'classes' is the definition.
(I know this)
'arts return thanks' is the wordplay.
'return' shows that the letters should be reversed in order.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'arts' in reverse letter order is 'stra'.
'stra'+'ta'='STRATA'
'in' is the link.
(Other definitions for strata that I've seen before include "Beds of sedimentary rock" , "Layers or thicknesses of earth" , "Layers of sedimentary rock" , "Layers or social classes" , "Geological levels" .)