The most common and most likely answer to this crossword clue is the 7 plus 6 letter word BERMUDA SHORTS
I've seen this crossword clue in Financial Times
B
E
R
M
U
D
A
S
H
O
R
T
S
Buddy explains!
The straight part
We know that "Informal wear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case informal wear = bermuda shorts.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.