The most common and most likely answer to this crossword clue is the 9 letter word GLAMORISE
I've seen this crossword clue in The Guardian
G
L
A
M
O
R
I
S
E
Buddy explains!

The straight part
We know that "dress up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dress up = glamorise.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 GLAMORISE mean?
GLAMORISE verb- To make or give the appearance of being glamorous.
- To glorify; to romanticize.