The most common and most likely answer to this crossword clue is the 12 letter word CAMIKNICKERS
I've seen this crossword clue in The Times
C
A
M
I
K
N
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C
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Buddy explains!

The straight part
We know that "Undergarment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case undergarment = camiknickers.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 CAMIKNICKERS mean?
CAMIKNICKERS noun- A women's undergarment covering the torso, worn for example under short dresses or slacks.