The most common and most likely answer to this crossword clue is the 5 plus 6 letter word HOUSE ARREST
I've seen this crossword clue in The Times
H
O
U
S
E
A
R
R
E
S
T
More clues leading to the result HOUSE ARREST
Buddy explains!
The straight part
We know that "One must stay in under this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one must stay in under this = house arrest.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.