Unacceptable group start (6)
I believe the answer is:
outset
'start' is the definition.
(I know that start can be written as outset)
'unacceptable group' is the wordplay.
'unacceptable' becomes 'out' (out can mean incorrect or inappropriate).
'group' becomes 'set' (both can mean a collection).
'out'+'set'='OUTSET'
(Other definitions for outset that I've seen before include "Kickoff time?" , "launch" , "Beginning, start" , "debut?" , "Fruit" .)