Slow sailor, dry on the outside (5)
I believe the answer is:
tardy
'slow' is the definition.
(tardy means running late)
'sailor dry on the outside' is the wordplay.
'sailor' becomes 'tar' (term for a sailor).
'on the outside' means to remove the middle letters.
'dry' with its middle removed is 'dy'.
'tar'+'dy'='TARDY'
(Other definitions for tardy that I've seen before include "Unpunctual" , "Dilatory" , "Late - slow" , "Rather late" , "Behindhand" .)