Long South American river (7)
I believe the answer is:
orinoco
'long south american river' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for orinoco that I've seen before include "Great river in the north of S America, flows through Venezuela" , "may be seen running through South America" , "South American river flowing into the Atlantic" , "large flow of water" , "Common character" .)