The most common and most likely answer to this crossword clue is the 11 letter word UNDERSKIRTS
I've seen this crossword clue in The Daily Telegraph
U
N
D
E
R
S
K
I
R
T
S
Buddy explains!

The straight part
We know that "11" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 11 = underskirts.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 UNDERSKIRTS mean?
UNDERSKIRTS noun- A skirt worn underneath another skirt; a petticoat.
- An under layer of a multi-layer gown over which outer skirts are draped.