Partner set up first-rate business groups (9)
I believe the answer is:
consortia
'business groups' is the definition.
(more than one consortium)
'partner set up first-rate' is the wordplay.
'partner' becomes 'consort' (synonyms).
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'first rate' becomes 'AI' (resembles Ac, old-fashioned slang for very good).
'ai' back-to-front is 'ia'.
'consort'+'ia'='CONSORTIA'
(Other definitions for consortia that I've seen before include "syndicates" , "Groups of companies" , "More than one commercial group" , "Groups of bodies" , "firms" .)