Final, conclusive (8)
I believe the answer is:
ultimate
I believe this clue is a double definition.
'final' is the first definition.
(similar in meaning)
'conclusive' is the second definition.
(I've seen this before)
(Other definitions for ultimate that I've seen before include "The last element of a series" , "Last or final in a series or process" , "Conclusive" , "Happening at the end of a process, final" , "Last or highest" .)