The most common and most likely answer to this crossword clue is the 6 letter word WARREN
W
A
R
R
E
N
More clues leading to the result WARREN
- Warn about edges of route in overcrowded area
- Architect welcoming return of artist's colony
- Put on guard to protect soldiers in densely populated area
- A rook nested in smaller species’ overcrowded area
- Bird circling a run in rabbits’ home
- Struggle about new maze
- Game at home here arranged during recent erecting
- Conflict about new underground network
- Combat resistance in Lyonnaise Buffet?
- The buck stops here
Buddy explains!

The straight part
We know that "underground network" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case underground network = warren.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 WARREN mean?
WARREN noun- The system of burrows where rabbits live.
- An enclosed piece of land set aside for breeding game, especially rabbits.
- A mazelike place of dark alleys etc in which it's easy to lose oneself; especially one that may be overcrowded.
- A heart-shaped hoe.