Old market's advertising poster's over entrance (12)

I believe the answer is:

billingsgate

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'old market's' is the definition.
'Billingsgate' can be an answer for 'market's' (London market). I am not certain of the 'old' bit.

'advertising poster's over entrance' is the wordplay.
'advertising poster's over' becomes 'billings' (I can't explain this - if you can you should believe this answer much more).
'entrance' becomes 'gate' (type of entrance).
'billings'+'gate'='BILLINGSGATE'

Can you help me to learn more?

(Other definitions for billingsgate that I've seen before include "London fish market" , "Swearing" .)

I've seen this clue in The Times.

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