Being the last element of a series (8)
I believe the answer is:
ultimate
'being the last element of a series' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ultimate that I've seen before include "Happening at the end of a process, final" , "Eventual" , "Last or final in a series or process" , "Utmost or extreme" , "Happening last" .)