Last in series is best (8)
I believe the answer is:
ultimate
This clue could be a double definition.
'last in series' is the first definition.
'ultimate' can be an answer for 'last' (I've seen this before). I am unsure of the 'in series' bit.
'best' is the second definition.
(I've seen this before)
'is' acts as a link.
Can you help me to learn more?
(Other definitions for ultimate that I've seen before include "Last, final or extreme" , "Mutilate in last possible way" , "Last possible, final" , "Last or concluding element" , "The last element of a series" .)