Problem about dreadful cant in sacred place (7)
I believe the answer is:
sanctum
'sacred place' is the definition.
(I've seen this before)
'problem about dreadful cant' is the wordplay.
'problem' becomes 'sum' (as in a mathematical problem).
'about' is an insertion indicator.
'dreadful' indicates an anagram.
'cant' anagrammed gives 'anct'.
'sum' going around 'anct' is 'SANCTUM'.
'in' acts as a link.
(Other definitions for sanctum that I've seen before include "Sacred place; private room" , "Private study" , "sacred space" , "SECOND PART OF 8" , "Private/sacred place" .)