The most common and most likely answer to this crossword clue is the 9 letter word GUERRILLA
I've seen this crossword clue in The Daily Telegraph
G
U
E
R
R
I
L
L
A
More clues leading to the result GUERRILLA
- Irregular rule a girl violated
- Somehow lure a girl, a freedom fighter?
- Irregular mostly out to install Left?
- Beastly sounding tactics
- Irregular glue ear? Dr will clean occasionally
- A month back, foul up badly in it? That’s irregular
- Rule a girl breaks for freedom fighter
- Irregular for the most part involved with Left?
- Bandit, familiar figure in jungle reportedly
- Irregular, finally swapping sides, shot another irregular
Buddy explains!
The straight part
We know that "one underground?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one underground? = guerrilla.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 GUERRILLA mean?
GUERRILLA noun- A soldier in a small independent group, fighting against the government or regular forces by surprise raids.
- A non-official war carried out by small independent groups; a guerrilla war.
- Relating to, using, or typical of guerrilla warfare, or its principles of small independent or non-official perpetrators.
- Relating to, using, or typical of guerrilla marketing.