Many a trading centre is central! (3)

I believe the answer is:

mid

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'is central' is the definition.
(thesaurus)

'many a trading centre' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'a trading centre' becomes 'id' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'm'+'id'='MID'

Can you help me to learn more?

(Other definitions for mid that I've seen before include "Of the central part" , "Denote the centre point" , "Dim sort of prefix" , "Prefix meaning centre" .)

I've seen this clue in the Evening Standard.

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