I’m in old city-state without proper approval (10)

I believe the answer is:

imprimatur

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'approval' is the definition.
(imprimatur is a kind of approval)

'i'm in old city-state without proper' is the wordplay.
'i'm' becomes 'i' (I've seen this in other clues).
'in old' becomes 'mat' (I can't explain this - if you can you should believe this answer much more).
'city state' becomes 'Ur' (ancient Sumerian city state).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'proper' becomes 'prim' (prim and proper).
'mat'+'ur'='matur'
'matur' enclosing 'prim' is 'mprimatur'.
'i'+'mprimatur'='IMPRIMATUR'

Can you help me to learn more?

(Other definitions for imprimatur that I've seen before include "church licence" , "Church approval" .)

I've seen this clue in The Times.

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