Final; decisive (8)
I believe the answer is:
ultimate
This clue is a double definition.
'final' is the first definition.
(similar in meaning)
'decisive' is the second definition.
(thesaurus)
(Other definitions for ultimate that I've seen before include "The final point" , "Last or final in a series or process" , "Utmost; last" , "Utmost or extreme" , "The last element of a series" .)