Mostly remain with chatty bird, showing capacity for endurance (7)
I believe the answer is:
stamina
'for endurance' is the definition.
(stamina is a kind of endurance)
'mostly remain with chatty bird' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'remain' becomes 'stay' (synonyms).
'with' means one lot of letters go next to another.
'chatty bird' becomes 'mina' (alternative spelling of myna. I am not sure about the 'chatty' bit.).
'stay' with its final letter taken away is 'sta'.
'sta'+'mina'='STAMINA'
'showing capacity' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for stamina that I've seen before include "energy to go on" , "Ability to maintain effort" , "toughness in > male parts" , "Vigour" , "Indefatigability" .)