A little mansion in Eaton Square (4)

I believe the answer is:

nine

'square' is the definition.
(nine is a square number)

'a little mansion in eaton' is the wordplay.
'a little' indicates a hidden word (a little bit of the clue).
'NINE' can be found hidden inside 'mansion in eaton'.

(Other definitions for nine that I've seen before include "Square next up after four" , "A stitch in time saves them" , "So many" , "SECOND PART OF 14" , "One's married partner" .)

I've seen this clue in The Independent.

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