The most common and most likely answer to this crossword clue is the 5 plus 5 letter word FANCY DRESS
I've seen this crossword clue in Financial Times
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More clues leading to the result FANCY DRESS
- Take a shine to groom in fun costume
- Like to line up disguise for ball?
- Deny scarf’s misplaced in party costume?
- Imagine groom’s costume
- Like the idea of force devoid of uniform or distinctive costume
- Like the look of doctorin party outfits
- Imagine pressure with no universal costume
- Desire clothing but not the ordinary sort
- What people wear during Halloween and kids contest
- Tend to like such a formal shirt perhaps as party wear?
Buddy explains!

The straight part
We know that "fun costume" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fun costume = fancy 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.
Crossword Answer definitions
FANCY DRESS noun- A costume, disguise to masquerade as something or someone else.
- Formalwear.