Tardier (6)
(Other definitions for slower that I've seen before include "Less able to understand" , "taking longer" , "Not so fast!" , "Being less intelligent" .)
'tardier' is the definition.
('late' can be a synonym of 'tardy')
This is all the clue.
(Other definitions for later that I've seen before include "after the rest?" , "succeeding" , "At a subsequent time" , "Tomorrow (or next week)?" , "At a time in the future" .)