The most common and most likely answer to this crossword clue is the 10 letter word SPELUNKING
I've seen this crossword clue in The Guardian
S
P
E
L
U
N
K
I
N
G
Buddy explains!
The straight part
We know that "Going underground" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case going underground = spelunking.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 SPELUNKING mean?
SPELUNKING verb- To explore caves.
- To explore a system in depth.
- The practice or hobby of exploring underground caverns.