The one bargain that couldn't be better (5)

I believe the answer is:

ideal

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'the one bargain that couldn't be better' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

'the one bargain' is the wordplay.
'the one' becomes 'i' (Roman numeral).
'bargain' becomes 'deal' (synonyms).
'i'+'deal'='IDEAL'

Can you help me to learn more?

(Other definitions for ideal that I've seen before include "Paragon" , "fanciful" , "Paradigm" , "Consummate" , "just the job" .)

I've seen this clue in the Evening Standard.

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