The most common and most likely answer to this crossword clue is the 6 letter word KNICKS
I've seen this crossword clue in The Times
K
N
I
C
K
S
More clues leading to the result KNICKS
Buddy explains!

The straight part
We know that "Little underwear" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case little underwear = knicks.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 KNICKS mean?
KNICKS noun- Knickers.
- Lycra pants (usually short) used by cyclists.