A complete change of opinion regarding the variety act (5-4)
I believe the answer is:
'a complete change of opinion' is the definition.
(I've seen this before)
'regarding the variety act' is the wordplay.
'regarding' becomes 'about' (I've seen this before).
'the variety act' becomes 'turn' (I can't explain this - if you can you should give a lot more credence to this answer).
Can you help me to learn more?
(Other definitions for about-turn that I've seen before include "Act of pivoting 180 degrees" , "Total change of direction" , "Change of opinion" .)