New car is not impregnable (4-4)
I believe the answer is:
cast-iron
'impregnable' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I can't see?
'new car is not' is the wordplay.
'new' indicates an anagram.
'car'+'is'+'not'='carisnot'
'carisnot' is an anagram of 'CAST-IRON'.
Can you help me to learn more?
(Other definitions for cast-iron that I've seen before include "Unassailable" , "Firm and unchangeable" , "Rigid, unyielding" , "Very strong" , "Unbreakable" .)