Humiliation in a flat below stairs? (9)

I believe the answer is:

abasement

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'humiliation' is the definition.
(synonyms)

'flat below stairs?' is the wordplay.
'flat' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'below stairs?' becomes 'basement'.
'a'+'basement'='ABASEMENT'

'in a' is the link.

Can you help me to learn more?

(Other definitions for abasement that I've seen before include "Crawling" , "Humiliating situation" , "Putting down" , "Degradation" , "will humiliate us" .)

I've seen this clue in The Guardian.

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