Swap for a whip (6)
I believe the answer is:
switch
I believe this is a double definition.
'swap' is the first definition.
(I know that swap can be written as switch)
'a whip' is the second definition.
(I know that switch is a type of whip)
'for' is the link.
(Other definitions for switch that I've seen before include "Light whip - light control" , "birch" , "that turns on the power" , "Change or cane" , "Contact breaker; change" .)