Second prize reveals mild performance (6,5)
I believe the answer is:
silver medal
'second prize' is the definition.
(eg at the Olympics)
'reveals mild performance' is the wordplay.
'performance' indicates an anagram.
'reveals'+'mild'='revealsmild'
'revealsmild' with letters rearranged gives 'SILVER MEDAL'.
(Other definitions for silver medal that I've seen before include "Sporting prize" , "Prize for second place" , "Olympic prize" , "What won't satisfy champion" , "Runner-up award" .)