Cause a complete reversal of circumstances (4,3,6)
I believe the answer is:
turn the tables
'cause 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 "Bring about a reversal of fortunes" , "Reverse fortune" , "Cause a reversal of fortune" , "effect complete reversal" , "spiritualists used to?" .)