Switch patroness when corrupt (9)
I believe the answer is:
transpose
'switch' is the definition.
(to transpose two things is to switch them around)
'patroness when corrupt' is the wordplay.
'when corrupt' is an anagram indicator (the original order is corrupted).
'patroness' with letters rearranged gives 'TRANSPOSE'.
(Other definitions for transpose that I've seen before include "Change places with patroness" , "Change position of patroness" , "Change the order or arrangement of" , "Change to a different key" , "Cause to change places" .)