Complete change of opinion concerning performer (5-4)
I believe the answer is:
about-turn
'complete change of opinion' is the definition.
(I've seen this before)
'concerning performer' is the wordplay.
'concerning' becomes 'about' (about can mean concerning or relating to).
'performer' becomes 'turn' ('turn' can mean a performer as well as their act).
'about'+'turn'='ABOUT-TURN'
(Other definitions for about-turn that I've seen before include "Change of opinion" , "Act of pivoting 180 degrees" , "Total change of direction" .)