Crossword Genius

Unassailable prisoner outside a prison (4-4)

Ross

I believe the answer is:

cast-iron

'unassailable' is the definition.
(I've seen this before)

'prisoner outside a prison' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'outside' is an insertion indicator (some letters go outside others).
'prison' becomes 'stir' (informal term for prison).
'a'+'stir'='astir'
'con' enclosing 'astir' is 'CAST-IRON'.

(Other definitions for cast-iron that I've seen before include "Extremely robust" , "Unassailable" , "Rigid, unyielding" , "Unbreakable" , "Firm and unchangeable" .)

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