Mind telling niece off? (12)
I believe the answer is:
intelligence
'mind' is the definition.
(thesaurus)
'telling niece off?' is the wordplay.
'off?' is an anagram indicator (I've seen 'off' mean this).
'telling'+'niece'='tellingniece'
'tellingniece' is an anagram of 'INTELLIGENCE'.
(Other definitions for intelligence that I've seen before include "Mental brightness" , "Spy reports" , "Covertly-gathered info" , "Information from spies" , "Covertly gathered information" .)