The most common and most likely answer to this crossword clue is the 8 letter word BLOWHOLE
I've seen this crossword clue in The Times
B
L
O
W
H
O
L
E
Buddy explains!
The straight part
We know that "escape route from tunnel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case escape route from tunnel = blowhole.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 BLOWHOLE mean?
BLOWHOLE noun- The spiracle, on the top of the head, through which cetaceans breathe.
- A vent for the escape of gas.
- A top-facing opening to a cavity in the ground very near an ocean's shore, leading to a marine cave from which wave water or bursts of air are expelled.
- An unintended cavity filled with air in a casting product.
- A vertical opening in the top of computer cases, that let hot air, primarily from the CPU heat sink, escape quickly.
- To fill or be filled with air in an unintended cavity.