Don't seem half-hearted, playing in final (7)
I believe the answer is:
endmost
'final' is the definition.
(I've seen this in another clue)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for endmost that I've seen before include "Farthest" , "Last in the line" , "extreme situation" , "final" , "Furthest away" .)