Endurance, staying power (7)
I believe the answer is:
stamina
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">endurance</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that stamina is a type of endurance)</span><br/ ><br/ >'<span class="explanation-format__original">staying power</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(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" .)