The most common and most likely answer to this crossword clue is the 7 letter word ZAMBEZI
I've seen this crossword clue in Financial Times
Z
A
M
B
E
Z
I
More clues leading to the result ZAMBEZI
- Unknown resin mostly found by Zulu half in river
- Last letter a doctor’s given to simple- sounding banker
- Lambnearly left, repeatedly turning into an unknown river
- South Africa award Zulu second bit of finance for river
- Maize British Honduras formerly scattered in river
- Zulus, one crossing briefly Orange River
Buddy explains!

The straight part
We know that "river" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case river = zambezi.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.