Most tardy (6)
'most tardy' is the definition.
('late' can be a synonym of 'tardy')
This is the entire clue.
(Other definitions for latest that I've seen before include "style of today" , "Most recent (news)" , "most behind time" , "Most recent or most tardy" , "Most up to the minute" .)
(Other definitions for slowest that I've seen before include "Most stupid" , "Least fast" , "Last in race" .)