Crete's terrible mystery (6)
I believe the answer is:
secret
'mystery' is the definition.
(thesaurus)
'crete's terrible' is the wordplay.
'terrible' indicates anagramming the letters.
'cretes' with letters rearranged gives 'SECRET'.
(Other definitions for secret that I've seen before include "It's not to be told" , "Security marking" , "Mystery" , "Unidentified" , "One not to be divulged" .)