The business of a mare being unfortunately out of a race (6)
I believe the answer is:
matter
'the business' is the definition.
(business can mean some matter or situation)
'mare being unfortunately out of a race' is the wordplay.
'being unfortunately' indicates anagramming the letters (a bad or unfortunate spelling).
'out of' is an insertion indicator (some letters on the outside of others).
'a race' becomes 'tt'.
'mare' anagrammed gives 'maer'.
'maer' enclosing 'tt' is 'MATTER'.
'of a' acts as a link.
(Other definitions for matter that I've seen before include "Be of significance" , "Substance; be important" , "That which occupies space in the visible world" , "Stuff - be of importance" , "Physicist giving name to frequency shift" .)