Shrewd as Nancy could be (5)
I believe the answer is:
canny
'shrewd' is the definition.
(both can mean having good judgement)
'nancy could be' is the wordplay.
'could be' indicates an anagram.
'nancy' is an anagram of 'CANNY'.
'as' is the link.
(Other definitions for canny that I've seen before include "Worldly-wise" , "Shrewd like Scotsman" , "Shrewd, knowing" , "Astute" , "Shrewd, clever" .)