Supporting best performance, to be factually correct (3,3,6)
I believe the answer is:
for the record
'to be factually correct' is the definition.
'supporting best performance' is the wordplay.
'supporting' becomes 'for'.
'best performance' becomes 'therecord'.
'for'+'therecord'='FOR THE RECORD'
(Other definitions for for the record that I've seen before include "(Said) to get facts straight" , "So that the true facts be known" , "To be strictly accurate" , "In order to get the facts straight" , "So that the facts are clear" .)