True stories about river fronts producing algae (8)
I believe the answer is:
Here is my best explanation:
'algae' is the definition.
(sargasso is a kind of algae)
'true stories about river fronts' is the subsidiary indication.
'true' becomes 'so' ('so' can be similar in meaning to 'true').
'stories' becomes 'sagas' (saga is a kind of story**).
'about' means one lot of letters goes inside another.
'river' becomes 'r'.
'fronts' means one lot of letters go next to another.
'sagas' going around 'r' is 'sargas'.
'so' put after 'sargas' is 'sargasso'.
'producing' is the link.
(Other definitions for sargasso that I've seen before include "'Sea in the North Atlantic, the only sea without shores (8)'", "Sea known for seaweed", "Region of Atlantic noted for masses of seaweed", "Name of N. Atlantic sea", "Atlantic sea surrounded by ocean currents", "Gas soars around the North Atlantic sea", "Sea, weedy part of the Atlantic", "Sea of west Atlantic noted for seaweed", "Seaweed" and "Sea whence eels migrate".)