Lowest score, ultimately, in final (5)
I believe the answer is:
least
'lowest' is the definition.
('least' can be a synonym of 'lowest')
'score ultimately in final' is the wordplay.
'score ultimately' becomes 'e' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'final' becomes 'last' ('last' can be a synonym of 'final').
'e' inserted inside 'last' is 'LEAST'.
Can you help me to learn more?
(Other definitions for least that I've seen before include "Most unimportant" , "Smallest, slightest" , "Opposite of most" , "Smallest, being stale" , "No more petty" .)