True stories about river fronts producing algae (8)
I believe the answer is:
sargasso
'algae' is the definition.
(sargasso is a kind of algae)
'true stories about river fronts' is the wordplay.
'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' enclosing 'r' is 'sargas'.
'so' put after 'sargas' is 'SARGASSO'.
'producing' acts as a link.
(Other definitions for sargasso that I've seen before include "'Sea in the North Atlantic, the only sea without shores (8)'" , "Seaweed giving name to a sea" , "Region of Atlantic noted for masses of seaweed" , "Name of N. Atlantic sea" , "Atlantic sea surrounded by ocean currents" .)