The most common and most likely answer to this crossword clue is the 9 plus 6 letter word REAR-VIEW MIRROR
I've seen this crossword clue in The Times
R
E
A
R
-
V
I
E
W
M
I
R
R
O
R
Buddy explains!
The straight part
We know that "river glances at this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case river glances at this = rear-view mirror.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.