One makes this successfully (4)
I believe the answer is:
good
'one' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for good that I've seen before include "Well done! Health-giving" , "benefit" , "Well-behaved, kind, or virtuous" , "promoting welfare" , "Fine" .)