Ceremony is to corrupt unknown self-importance (9)

I believe the answer is:

pomposity

'self-importance' is the definition.
(I've seen this before)

'ceremony is to corrupt unknown' is the wordplay.
'ceremony' becomes 'pomp' (I've seen this before).
'corrupt' indicates anagramming the letters.
'unknown' becomes 'y' (algebra).
'is'+'to'='isto'
'isto' anagrammed gives 'osit'.
'pomp'+'osit'+'y'='POMPOSITY'

(Another definition for pomposity that I've seen is " Self-importance".)

I've seen this clue in The Daily Express.

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