Impudent, but shy about certain knowledge at first (5)
I believe the answer is:
cocky
'impudent' is the definition.
(thesaurus)
'shy about certain knowledge at first' is the wordplay.
'shy' becomes 'coy' (coy can mean shy or modest).
'about' indicates putting letters inside.
'at first' indicates taking the first letters.
The first letters of 'certain knowledge' is 'ck'.
'coy' enclosing 'ck' is 'COCKY'.
'but' is the link.
(Other definitions for cocky that I've seen before include "Cheekily conceited" , "Arrogant (slang)" , "Boldly self-confident" , "Overweening" , "Cheekily confident" .)