One of eight finally finishing early (5)
I believe the answer is:
atlas
'one of eight' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'finally finishing early' is the wordplay.
'finally' becomes 'at last' (synonyms).
'finishing early' means to remove the last letter (the word finishes earlier than normal).
'atlast' with its final letter taken away is 'ATLAS'.
Can you help me to learn more?
(Other definitions for atlas that I've seen before include "One hugely burdened" , "Reference work" , "Ballistic missile" , "Sky-toting Titan" , "Its architectural name is telamon" .)