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 anagramming the letters.
'nancy' anagrammed gives 'CANNY'.
'as' acts as a link.
(Other definitions for canny that I've seen before include "Worldly-wise" , "Shrewd like Scotsman" , "Shrewd, knowing" , "Astute" , "Shrewd, clever" .)