Fairground ride is to revert to previous state (10)
I believe the answer is:
switchback
I believe this is a double definition.
'fairground ride' is the first definition.
(switchback can mean a roller coaster)
'revert to previous state' is the second definition.
(to switch back to an earlier state)
'is to' acts as a link.
(Other definitions for switchback that I've seen before include "uneven way" , "Up-and-down road" , "fairground attraction" , "ups and downs" , "big dipper" .)