Society levels central to Boston, Tirana and Ottawa (6)

I believe the answer is:

strata

I'm a little stuck... Click here to teach me more about this clue!

'society levels' is the definition.
'strata' can be an answer for 'levels' (I've seen this in another clue). I am not sure about the 'society' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for strata that I've seen before include "Geological levels" , "Layers as of rocks in earth's crust" , "Beds of sedimentary rock" , "Layers into which rocks are divided" , "Tiers of thicknesses" .)

I've seen this clue in The Irish Times.

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