The most common and most likely answer to this crossword clue is the 6 letter word AMAZON
I've seen this crossword clue in The Guardian
A
M
A
Z
O
N
More clues leading to the result AMAZON
- Flower a maiden attached to a short belt
- Almost cause sensation, performing as female warrior
- Single-breasted styles to suit this warrior?
- Puzzle briefly referring to river
- A puzzle not completed — needing old name for flower?
- Flower shop online here?
- No limits to damp and haze on river
- Giantess, great runner
- Edges of aluminium razor shaved face of noble warrior
- English extremists kidnap mother on 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 = amazon.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.
What does AMAZON mean?
AMAZON noun- A tall, strong, athletic woman.