The most common and most likely answer to this crossword clue is the 8 letter word GLACIERS
I've seen this crossword clue in The Daily Telegraph
G
L
A
C
I
E
R
S
Buddy explains!
The straight part
We know that "what must be going down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what must be going down = glaciers.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 GLACIERS mean?
GLACIERS noun- A large body of ice which flows under its own mass, usually downhill.