Impregnability of Superman figure wearing thick cape (8)
I believe the answer is:
fastness
'impregnability of superman figure' is the definition.
The definition and answer can be both human attributes as well as being singular nouns.
Maybe there's a link between them I don't understand?
'wearing thick cape' is the wordplay.
'wearing thick' becomes 'fast' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cape' becomes 'ness' (synonyms).
'fast'+'ness'='FASTNESS'
Can you help me to learn more?
(Other definitions for fastness that I've seen before include "quality of not fading" , "Stronghold or fortress" , "Strongly fortified defensive place" , "Secure refuge" , "speed?" .)