The most common and most likely answer to this crossword clue is the 4 letter word QUOD
Q
U
O
D
Buddy explains!
The straight part
We know that "British prison" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case british prison = quod.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 QUOD mean?
QUOD noun- A quadrangle or court, as of a prison; a prison.
- Confinement in a prison.