Slow road engulfed by Scottish river (5)
I believe the answer is:
tardy
'slow' is the definition.
(tardy means running late)
'road engulfed by scottish river' is the wordplay.
'road' becomes 'rd' (abbreviation).
'engulfed by' means one lot of letters goes inside another.
'scottish river' becomes 'tay' (the River Tay in Scotland).
'rd' put within 'tay' is 'TARDY'.
(Other definitions for tardy that I've seen before include "Belated" , "Late - slow" , "Late, delayed" , "Backward" , "Behindhand" .)