Prison con practically amassing crust (5)

I believe the answer is:

gulag

I'm a little stuck... Click here to teach me more about this clue!

'prison' is the definition.
(gulag is a kind of prison)

I don't understand how the rest of the clue works.

(Other definitions for gulag that I've seen before include "Book by Solzhenitsyn 'The . . . . . Archipelago' about detention camps in USSR" , "USSR system of labour camps" , "where state's enemies were held" , "place of detention" , "Russian labour camp system" .)

I've seen this clue in the Sydney Morning Herald.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.