An upright transaction is best (5)

I believe the answer is:

ideal

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

'an upright transaction' is the wordplay.
'an' becomes 'i' (Roman numeral for one).
'upright transaction' becomes 'deal' (deal is a kind of transaction. I am not sure about the 'upright' bit.).
'i'+'deal'='IDEAL'

Can you help me to learn more?

(Other definitions for ideal that I've seen before include "Standard of perfection or excellence" , "without a flaw" , "Model of excellance" , "Paragon" , "superb specimen" .)

I've seen this clue in the Evening Standard.

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