Extremely robust, a prison? Prisoner gets outside it! (4-4)
I believe the answer is:
cast-iron
'extremely robust' is the definition.
(I've seen this before)
'a prison? prisoner gets outside it' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
'prison?' could be 'stir' (informal term for prison) and 'stir' is found in the answer.
'prisoner' could be 'con' (abbreviation for 'convict') and 'con' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cast-iron that I've seen before include "Very strong" , "Unbreakable" , "Unassailable" , "Firm and unchangeable" , "Rigid, unyielding" .)