Runner is more able to survive having run for the day (7)
I believe the answer is:
harrier
'runner' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
(Other definitions for harrier that I've seen before include "Long distance runner" , "may get on your back?" , "Short-winged hawk" , "that flies!" , "jump jet" .)