Not old (3)
'not old' is the definition.
('new' is the opposite to 'old')
This is the entire clue.
(Other definitions for new that I've seen before include "Unfamiliar, restored" , "Unconventional" , "Recently made, etc" , "Not existing before" , "Contemporary" .)
'not old' is the definition.
('young' is the opposite to 'old')
This is all the clue.
(Other definitions for young that I've seen before include "Youthful" , "Juvenile" , "Mormon leader" , "Wilder scientist was described thus" , "issue" .)