Unassailable prisoner outside a prison (4-4)
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' indicates putting letters inside.
'prison' becomes 'stir' (informal term for prison).
'a'+'stir'='astir'
'con' going around 'astir' is 'CAST-IRON'.
(Other definitions for cast-iron that I've seen before include "Extremely robust" , "Unassailable" , "Rigid, unyielding" , "Unbreakable" , "Firm and unchangeable" .)