Past fairground attraction set aside (8)
I believe the answer is:
override
'set aside' is the definition.
(thesaurus)
'past fairground attraction' is the wordplay.
'past' becomes 'over' (both can mean finished).
'fairground attraction' becomes 'ride' (I've seen this in another clue).
'over'+'ride'='OVERRIDE'
(Other definitions for override that I've seen before include "Use one's authority to reject or cancel a decision" , "Annul" , "Be more important than" , "Supersede" , "Counteract the normal operation" .)