Crossword Genius

A middling, sound business (6)

Ross

I believe the answer is:

affair

I'm a little stuck... Click here to teach me more about this clue!

'business' is the definition.
(I've seen this before)

'a middling sound' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
'middling' could be 'fair' (synonyms) and 'fair' is found within the answer.
A single letter 'f' remains which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for affair that I've seen before include "Raffia (anag)" , "A romantic intrigue" , "Singular business" , "A function or a romance" , "Matter or concern, perhaps of the heart" .)

I've seen this clue in the Sydney Morning Herald.
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