Telling niece about brainpower (12)
I believe the answer is:
intelligence
'brainpower' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'telling niece about' is the wordplay.
'about' indicates anagramming the letters.
'telling'+'niece'='tellingniece'
'tellingniece' is an anagram of 'INTELLIGENCE'.
Can you help me to learn more?
(Other definitions for intelligence that I've seen before include "Spy reports" , "Information from spies" , "Mental brightness" , "Ken" , "Dope" .)