What does the cryptic crossword clue "From Borstal, a ghastly prison (6)" mean?

The definition is "prison". The wordplay uses a hidden word. Hidden in "from borSTAL, A Ghastly prison" The answer is STALAG.

From Borstal, a ghastly prison (6)

Guardian Cryptic #29130 | 3 Down | 2023-07-24 flower