Classes engaged in history and philosophy, say, on reflection (6)
I believe the answer is:
strata
'classes' is the definition.
(I've seen this before)
'engaged in history and philosophy say on reflection' is the wordplay.
'engaged in history' becomes 'ta' (I can't explain this - if you can you should believe this answer much more).
'and' says to put letters next to each other.
'philosophy say' becomes 'arts' (philosophy is a kind of arts).
'on reflection' shows that the letters should be reversed in order.
'arts' reversed gives 'stra'.
'ta' put after 'stra' is 'STRATA'.
Can you help me to learn more?
(Other definitions for strata that I've seen before include "Geological layers" , "Layers of sedimentary rock" , "Layers or thicknesses of earth" , "levels of society" , "Tiers of thicknesses" .)