Bring about a complete reversal of circumstances (4,3,6)
I believe the answer is:
turn the tables
'bring about a complete reversal of circumstances' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for turn the tables that I've seen before include "Cause a reversal of fortune" , "Make a complete reversal" , "Gain an advantage from an unfavourable position" , "Bring about a complete reversal of circumstances" , "Change from losing to winning" .)