Red Sea rising over a source of sweetness (8)
I believe the answer is:
demerara
'source of sweetness' is the definition.
(I've seen this before)
'red sea rising over a' is the wordplay.
'red' becomes 'rare' (as in a rare steak).
'sea' becomes 'med' (short for Mediterranean).
'rising' shows that the letters should be reversed in order (in a down clue, letters go up).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'rare'+'med'='raremed'
'raremed' reversed gives 'demerar'.
'demerar'+'a'='DEMERARA'
(Other definitions for demerara that I've seen before include "sweetener" , "(Light-brown) sugar" , "Variety of sugar" , "Source of sweetness" , "sugar - and alcohol" .)