The most common and most likely answer to this crossword clue is the 9 letter word ONE-PIECE
I've seen this crossword clue in Independent
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More clues leading to the result ONE-PIECE
- Sort of accoutrement was successful in war and what followed broadcast
- Slimmer’s chocolate ration, perhaps, if she’s to look good in it!
- Pee on ice dancing cozzie
- On vast unending beaches, regularly displaying a type of swimsuit
- .... old relative using extremely elegant and soft clothing
- Start to eat pastry having at some previous point worn swimsuit?
Buddy explains!

The straight part
We know that "a type of swimsuit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a type of swimsuit = one-piece.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 ONE-PIECE mean?
ONE-PIECE noun- A one-piece article of clothing, especially a swimsuit.
- Composed of a single integral unit or so appearing.