Opposite of late (5)
I believe the answer is:
early
'opposite of late' is the definition.
('early' is the opposite to 'late')
This is the entire clue.
(Other definitions for early that I've seen before include "Before the ETA" , "before anyone else?" , "Infancy, when Wordsworth had 5 across of 5 down" , "Before appointed time" , "Before the usual time" .)