One stays with it (7)
I believe the answer is:
stamina
'one stays' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for stamina that I've seen before include "capacity to endure" , "Enduring energy" , "Ability to maintain effort" , "energy to go on" , "Staying power, ability to withstand prolonged strain" .)