Telling niece off for sharpness (12)
I believe the answer is:
intelligence
'sharpness' is the definition.
'telling niece off' is the wordplay.
'off' indicates an anagram.
'telling'+'niece'='tellingniece'
'tellingniece' with letters rearranged gives 'INTELLIGENCE'.
'for' is the link.
(Other definitions for intelligence that I've seen before include "Mind; news" , "Covertly-gathered info" , "army section?" , "Secret information" , "Covert information" .)