Overconfident birds on river (8)
I believe the answer is:
cocksure
'overconfident' is the definition.
(I know that overconfident can be written as cocksure)
'birds on river' is the wordplay.
'birds' becomes 'cocks' (cock is a kind of bird).
'on' is a charade indicator (letters next to each other).
'river' becomes 'ure' (river in Yorkshire).
'cocks'+'ure'='COCKSURE'
(Other definitions for cocksure that I've seen before include "Showing arrogance" , "Annoyingly self-confident" , "The fowl begins to be offensively certain." , "Arrogantly certain" , "Arrogantly confident" .)