Substitute second one of three Shakespearean sisters (6)
I believe the answer is:
switch
'substitute' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for switch that I've seen before include "On/off device" , "Change or exchange" , "Change course" , "Change round" , "that turns on the power" .)