Start as an unacceptable group (6)
I believe the answer is:
outset
'start' is the definition.
('outset' can be a synonym of 'start')
'an unacceptable group' is the wordplay.
'an unacceptable' becomes 'out' (I've seen this in other clues).
'group' becomes 'set' (set is a kind of group**).
'out'+'set'='OUTSET'
'as' acts as a link.
(Other definitions for outset that I've seen before include "origin" , "Fruit" , "launch" , "Kickoff time?" , "Starting point" .)