A bird under good man showing energy (7)
I believe the answer is:
stamina
'energy' is the definition.
(thesaurus)
'a bird under good man' is the wordplay.
'bird' becomes 'mina' (mina is a kind of bird**).
'under' means one lot of letters go next to another.
'good man' becomes 'st'.
'a'+'mina'='amina'
'amina' put after 'st' is 'STAMINA'.
'showing' acts as a link.
(Other definitions for stamina that I've seen before include "bottom" , "resistance" , "having more to call on" , "Parts of flowers" , "Sustained energy" .)