Mainstay gets tip-off as cause of lasting ability (7)
I believe the answer is:
stamina
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for stamina that I've seen before include "Parts of flowers" , "Endurance - staying power" , "having more to call on" , "legs" , "Energy and staying power" .)