Not late but almost losing lead (5)
I believe the answer is:
early
'not late' is the definition.
('early' is the opposite to 'late')
'almost losing lead' is the wordplay.
'almost' becomes 'nearly' (synonyms).
'losing lead' means to remove the first letter.
'nearly' with its first letter taken away is 'EARLY'.
'but' is the link.
(Other definitions for early that I've seen before include "Immature" , "sooner than expected" , "Untimely" , "Opening" , "near beginning" .)