One old port that will never improve! (5)

I believe the answer is:

ideal

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'port that will never improve' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?

'one old' is the wordplay.
'one' becomes 'i' (Roman numeral).
'old' becomes 'deal' (I can't justify this - if you can you should believe this answer much more).
'i'+'deal'='IDEAL'

Can you help me to learn more?

(Other definitions for ideal that I've seen before include "Consummate" , "Embodying perfection" , "'Perfect, model (5)'" , "example" , "Best possible" .)

I've seen this clue in the Evening Standard.

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