Extremely sensitive setter cop tickled? (3-6)
I believe the answer is:
top-secret
'extremely sensitive' is the definition.
(as in highly sensitive information)
'setter cop tickled?' is the wordplay.
'tickled?' is an anagram indicator (I've seen this in other clues).
'setter'+'cop'='settercop'
'settercop' is an anagram of 'TOP-SECRET'.
(Other definitions for top-secret that I've seen before include "Most confidential , not to be leaked on any account" , "Very hush-hush" , "Highly confidential" , "(Of information) strictly not for public consumption" , "Erect post (anag)" .)