The most common and most likely answer to this crossword clue is the 8 plus 5 letter word ACADEMIC DRESS
I've seen this crossword clue in The Times
A
C
A
D
E
M
I
C
D
R
E
S
S
Buddy explains!
The straight part
We know that "Don clothing?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case don clothing? = academic dress.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.