Overconfident bird? Certainly! (8)
I believe the answer is:
cocksure
'overconfident' is the definition.
(I know that overconfident can be written as cocksure)
'bird? certainly' is the wordplay.
'bird?' becomes 'cock' (cock is a kind of bird).
'certainly' becomes 'sure' (synonyms).
'cock'+'sure'='COCKSURE'
(Other definitions for cocksure that I've seen before include "Foolishly over-confident" , "Annoyingly self-confident" , "The fowl begins to be offensively certain." , "Very confident like male fowl" , "Showing arrogance" .)