Slow sailor, extremely dilatory (5)
I believe the answer is:
tardy
'slow' is the definition.
(tardy means running late)
'sailor extremely dilatory' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'dilatory' with its centre taken out is 'dy'.
'tar'+'dy'='TARDY'
(Other definitions for tardy that I've seen before include "Backward" , "Rather late" , "Late - slow" , "Unpunctual" , "Late, delayed" .)