Uncle doesn't finish, and youth doesn't start, being coarse (7)
I believe the answer is:
uncouth
'coarse' is the definition.
(I know that coarse can be written as uncouth)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for uncouth that I've seen before include "Boorish, clumsy in manner" , "in an awkward way" , "Ill-bred and coarse" , "Loutish" , "Crude" .)