The most common and most likely answer to this crossword clue is the 10 letter word TRACKSUITS
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Buddy explains!
The straight part
We know that "casual apparel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case casual apparel = tracksuits.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 TRACKSUITS mean?
TRACKSUITS noun- A garment, usually consisting of a top and trousers (commonly known as tracksuit bottoms) worn as an outer layer by participants in sporting events such as athletics. The tracksuit is usually designed to be easily removed or replaced, before or after competing. Tracksuits have also been adopted in some cultures as leisurewear.