The most common and most likely answer to this crossword clue is the 5 letter word TWEED
I've seen this crossword clue in The Guardian
T
W
E
E
D
More clues leading to the result TWEED
- Our group impressed by Heath, say, in river
- Water that flows close to port and dock?
- Quaint end of old cloth
- Cloth in river at Berwick
- Time to remove unwanted plants in river
- Daughter's after sentimentally pretty fabric
- Border flower, sweetly pretty, died
- Weaver is the source for this northern river
- Used aid for driving across western river
- Source of Thames produced water for 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 = tweed.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 TWEED mean?
TWEED noun- A coarse woolen fabric used for clothing.