Outstrip when restored to health (6)
I believe the answer is:
'outstrip' is the definition.
(bettering is a kind of outstriping)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for better that I've seen before include "no longer sick" , "Superior; gambler" , "Gambler; more effective" , "Preferable" , "More desirable; gambler" .)