The most common and most likely answer to this crossword clue is the 7 letter word RAFTING
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Buddy explains!

The straight part
We know that "going down river" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case going down river = rafting.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 RAFTING mean?
RAFTING verb- To convey on a raft.
- To make into a raft.
- To travel by raft.
- To dock (toolbars, etc.) so that they share horizontal or vertical space.
- The sport of guiding a raft while descending a river, especially through rapids known as white water rafting.