The most common and most likely answer to this crossword clue is the 8 plus 7 letter word SWIMMING COSTUME
I've seen this crossword clue in Financial Times
S
W
I
M
M
I
N
G
C
O
S
T
U
M
E
Buddy explains!
The straight part
We know that "Trunks" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case trunks = swimming costume.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.