The most common and most likely answer to this crossword clue is the 5 plus 5 letter word FROCK COATS
I've seen this crossword clue in The Guardian
F
R
O
C
K
C
O
A
T
S
Buddy explains!
The straight part
We know that "formal wear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case formal wear = frock coats.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.