Ostentation shown by Medicis regularly in grand old city (7)

I believe the answer is:

pompeii

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'old city' is the definition.
(I've seen this before)

'ostentation shown by medicis regularly' is the wordplay.
'ostentation' becomes 'pomp' (I've seen this before).
'shown by' means one lot of letters go next to another.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'medicis' are 'eii'.
'pomp'+'eii'='POMPEII'

'in grand' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.

Can you help me to learn more?

(Other definitions for pompeii that I've seen before include "Roman city south-east of Naples" , "Ancient town near Naples buried by volcanic ash" , "Lava-buried Roman city" , "Town buried by Vesuvius 1st century a.d." , "City buried by volcanic eruption, AD 79" .)

I've seen this clue in The Everyman puzzle in The Observer.

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