She's relatively nice about an early start or late finish (5)
I believe the answer is:
niece
'late finish' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'she's relatively nice about an early start' is the wordplay.
I cannot really understand how this works, but
an anagram of 'nice' is 'niec' which is present in the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'or' is the link.
Can you help me to learn more?
(Other definitions for niece that I've seen before include "Kinswoman" , "Female relation" , "A sibling's child" , "Son's cousin" , "Brother or sister's daughter" .)