Noticed final terrible secret (12)
I believe the answer is:
confidential
'secret' is the definition.
(I know that secret can be written as confidential)
'noticed final terrible' is the wordplay.
'terrible' indicates an anagram.
'noticed'+'final'='noticedfinal'
'noticedfinal' is an anagram of 'CONFIDENTIAL'.
(Other definitions for confidential that I've seen before include "not expecting publication" , "Classified" , "Intended to be kept secret" , "Like T.S. Eliot's clerk" , "Graham Greene's agent" .)