The most common and most likely answer to this crossword clue is the 8 letter word TAILCOAT
I've seen this crossword clue in Financial Times
T
A
I
L
C
O
A
T
More clues leading to the result TAILCOAT
Buddy explains!
The straight part
We know that "something more formal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case something more formal = tailcoat.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 TAILCOAT mean?
TAILCOAT noun- A formal evening jacket with an extended back panel; a dress coat.
- Any coat with similar tails.