A middling, sound business (6)
I believe the answer is:
affair
'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" .)