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 Financial Times
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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 "Slimmer’s chocolate ration, perhaps, if she’s to look good in it!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slimmer’s chocolate ration, perhaps, if she’s to look good in it! = 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.