Simon restored the facade of an imposing building (7)

I believe the answer is:

mansion

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'building' is the definition.
(I have seen 'Large building' mean 'mansion' so perhaps 'building' could also mean 'mansion')

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mansion that I've seen before include "Manor house" , "Grand house" , "housing" , "Huge abode" , "Handsome house as for Lord Mayor" .)

I've seen this clue in the Evening Standard.

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