The most common and most likely answer to this crossword clue is the 9 letter word CAMISOLES
C
A
M
I
S
O
L
E
S
Buddy explains!

The straight part
We know that "underwear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case underwear = camisoles.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 CAMISOLES mean?
CAMISOLES noun- A short, sleeveless undergarment worn by women underneath a blouse, or as a form of short négligée.
- A straitjacket.
- A light jacket with sleeves.