Endurance, staying power (7)
I believe the answer is:
stamina
This clue is a double definition.
'endurance' is the first definition.
(I know that stamina is a type of endurance)
'staying power' is the second definition.
(synonyms)
(Other definitions for stamina that I've seen before include "The ability to sustain prolonged physical or mental effort" , "Endurance, strength to stay the course" , "Endurance - staying power" , "Vigour" , "resistance" .)