Prison block needing the ultimate in security (6)

I believe the answer is:

chokey

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

'prison' is the definition.
(I know that chokey is a type of prison)

'block needing the ultimate in security' is the wordplay.
'block' becomes 'choke' (choking is a kind of blocking).
'needing' is a charade indicator (letters next to each other).
'the ultimate in security' becomes 'y' (I can't justify this - if you can you should believe this answer much more).
'choke'+'y'='CHOKEY'

Can you help me to learn more?

(Another definition for chokey that I've seen is " Prison (slang)".)

I've seen this clue in The Guardian.

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.