Good man with a bird showing vigour (7)
I believe the answer is:
stamina
'vigour' is the definition.
(I've seen this before)
'good man with a bird' is the wordplay.
'good man' becomes 'st'.
'with' is a charade indicator (letters next to each other).
'bird' becomes 'mina' (mina is a kind of bird**).
'st'+'a'+'mina'='STAMINA'
'showing' is the link.
(Other definitions for stamina that I've seen before include "bottom" , "Staying-power, endurance" , "resistance" , "Vigour" , "legs" .)