The most common and most likely answer to this crossword clue is the 12 letter word UNDERDRESSES
I've seen this crossword clue in The Daily Telegraph
U
N
D
E
R
D
R
E
S
S
E
S
Buddy explains!
The straight part
We know that "Is clothed simply" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case is clothed simply = underdresses.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 UNDERDRESSES mean?
UNDERDRESSES verb- To dress in insufficiently warm clothes.
- To dress in insufficiently formal clothes.