The most common and most likely answer to this crossword clue is the 6 plus 4 letter word PADDED CELL
I've seen this crossword clue in The Guardian
P
A
D
D
E
D
C
E
L
L
Buddy explains!
The straight part
We know that "which prevents self-harm" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case which prevents self-harm = padded cell.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.