But not one of the last people you’d expect to win! (8)
I believe the answer is:
'but not one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'the last people you'd expect to win' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'last' could be 'final' (synonyms) and 'final' is present in the answer.
The remaining letters 'is' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for finalist that I've seen before include "Competitor who has beaten all except one" , "So far unbeaten competitor" , "He qualified" , "Competitor who reaches the last stage" , "One reaching last match" .)