The most common and most likely answer to this crossword clue is the 5 letter word CLOAK
I've seen this crossword clue in The Times
C
L
O
A
K
Buddy explains!

The straight part
We know that "underground" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case underground = cloak.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 CLOAK mean?
CLOAK noun- A long outer garment worn over the shoulders covering the back; a cape, often with a hood.
- A blanket-like covering, often metaphorical.
- That which conceals; a disguise or pretext.
- A text replacement for an IRC user's hostname or IP address, making the user less identifiable.
- To cover as with a cloak.
- To hide or conceal.
- To render or become invisible via futuristic technology.