Dead slow (4)
I believe the answer is:
late
This clue is a double definition.
'dead' is the first definition.
('late' can be similar in meaning to 'dead')
'slow' is the second definition.
(thesaurus)
(Other definitions for late that I've seen before include "Dead or not punctual" , "near the end" , "Dead - tardy" , "Running behind time" , "Delayed; dead" .)