Even defeat can show how good one can be? (5,4)
I believe the answer is:
level best
'show how good one can be?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'even defeat' is the wordplay.
'even' becomes 'level' (synonyms).
'defeat' becomes 'best' (besting is a kind of defeating).
'level'+'best'='LEVEL BEST'
'can' acts as a link.
Can you help me to learn more?
(Other definitions for level best that I've seen before include "Utmost effort" , "Hardest" , "Maximum efforts" , "joint winners?" .)