Impenetrable contents of hidden secret (5)
I believe the answer is:
dense
'impenetrable' is the definition.
(I know that impenetrable can be written as dense)
'contents of hidden secret' is the wordplay.
'contents of' indicates the answer is hidden within the clue.
'DENSE' is hidden within 'hidden secret'.
(Other definitions for dense that I've seen before include "Close knit" , "Obtuse - impenetrable" , "Closely compacted" , "Stupid; compacted" , "Not very bright" .)