The most common and most likely answer to this crossword clue is the 2 plus 4 plus 3 letter word IN FULL RIG
I've seen this crossword clue in The Guardian
I
N
F
U
L
L
R
I
G
Buddy explains!
The straight part
We know that "wearing ceremonial clothes" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case wearing ceremonial clothes = in full rig.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.