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Lost for words in an open, old prison for five hundred (6)

Ross

I believe the answer is:

amazed

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'lost for words' is the definition.
Although both the answer and definition are past participle verbs, I can't see how one could define the other.

'an open old prison for five hundred' is the wordplay.
'open' indicates taking the first letters (I've seen 'opens' mean this).
'old prison' becomes 'maze' (Northern Irish prison).
'for' says to put letters next to each other.
'five hundred' becomes 'D' (Roman numeral).
The initial letter of 'an' is 'a'.
'a'+'maze'+'d'='AMAZED'

'in' is the link.

Can you help me to learn more?

(Other definitions for amazed that I've seen before include "Greatly surprised or astonished" , "knocked for six" , "was stunning" , "Dumbfounded" , "Greatly surprised, astounded" .)

I've seen this clue in the Irish Times (Crosaire) and The Irish Times.
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