Showing confidence about city firm (6)

I believe the answer is:

secure

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'firm' is the definition.
(similar in meaning)

'showing confidence about city' is the wordplay.
'showing confidence' becomes 'sure' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'city' becomes 'EC' (postcode for the City of London).
'sure' enclosing 'ec' is 'SECURE'.

Can you help me to learn more?

(Other definitions for secure that I've seen before include "Peg" , "Free from danger or risk" , "'Safe, confident (6)'" , "Free from danger and financially sound" , "Safe, free from danger" .)

I've seen this clue in The Times.

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