The most common and most likely answer to this crossword clue is the 13 letter word LONG-DISTANCE
I've seen this crossword clue in The Guardian
L
O
N
G
-
D
I
S
T
A
N
C
E
Buddy explains!

The straight part
We know that "trunk" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trunk = long-distance.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 LONG-DISTANCE mean?
LONG-DISTANCE adjective- Over a great length or distance.
- Referring to a non-local telephone call; a toll call.
- Referring to a romantic relationship: between two people with a considerable distance between them.