The most common and most likely answer to this crossword clue is the 6 letter word RAPIDS
I've seen this crossword clue in The Times
R
A
P
I
D
S
More clues leading to the result RAPIDS
- Charges across quiet part of river
- Attacks engulfing quiet area of running water
- White-tipped parts of a flower?
- Charge is about old penny for what intrepid canoeist enjoys
- Pressure to stop attacks which could mean danger down-river
- Fish caught by rafter thrown back where rafter has fun?
- Patrol leader engulfed by storms in cataract
- Breaks into houses quietly in rough area
- Hard to avoid plant-sucking insects by river – in this section?
- Rushing water rushes maintaining pressure
Buddy explains!

The straight part
We know that "part of river" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of river = rapids.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 RAPIDS mean?
RAPIDS noun- (often in the plural) a rough section of a river or stream which is difficult to navigate due to the swift and turbulent motion of the water.
- A burst of rapid fire.