Go on or come off (5)
I believe the answer is:
occur
I believe this is a double definition.
'go on' is the first definition.
(synonyms)
'come off' is the second definition.
(I've seen this before)
(Other definitions for occur that I've seen before include "exist" , "appear" , "Turn up" , "Happen, take place" , "Befall" .)