Unacceptable to arrive at a conclusion (7)
I believe the answer is:
outcome
'a conclusion' is the definition.
(I know that outcome is a type of conclusion)
'unacceptable to arrive' is the wordplay.
'unacceptable' becomes 'out' (I've seen this in other clues).
'to arrive' becomes 'come' (coming is a kind of arriving).
'out'+'come'='OUTCOME'
'at' acts as a link.
(Other definitions for outcome that I've seen before include "consequences" , "'Result, upshot (7)'" , "answer" , "End result" , "Cute moo (anag.)" .)