Telling niece about secret enemy information? (12)
I believe the answer is:
intelligence
'secret enemy information?' is the definition.
(eg military intelligence)
'telling niece about' is the wordplay.
'about' indicates anagramming the letters.
'telling'+'niece'='tellingniece'
'tellingniece' anagrammed gives 'INTELLIGENCE'.
(Other definitions for intelligence that I've seen before include "Mind; news" , "Information from spies" , "Mental brightness" , "Covertly gathered information" , "Secret information" .)