The most common and most likely answer to this crossword clue is the 9 letter word UNDERPASS
I've seen this crossword clue in The Times
U
N
D
E
R
P
A
S
S
More clues leading to the result UNDERPASS
Buddy explains!
The straight part
We know that "tunnel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tunnel = underpass.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 UNDERPASS mean?
UNDERPASS noun- A passage that crosses a road, railroad or similar obstacle in a tunnel underneath it.