Telling niece dubious facts (12)
I believe the answer is:
intelligence
'facts' is the definition.
(thesaurus)
'telling niece dubious' is the wordplay.
'dubious' indicates an anagram.
'telling'+'niece'='tellingniece'
'tellingniece' is an anagram of 'INTELLIGENCE'.
(Other definitions for intelligence that I've seen before include "Military information" , "Secret information" , "Covertly-gathered info" , "Covert information" , "Mind; news" .)