Stop taking part in open days (3)
I believe the answer is:
end
'stop' is the definition.
(I know that stop is a more specific form of break up)
'part in open days' is the wordplay.
'part in' indicates a hidden word.
'END' is hidden within 'open days'.
'taking' is the link.
(Other definitions for end that I've seen before include "Exterminate - object" , "result" , "Terminate in den" , "For Chelsea or Villa, it's a" , "Finish, wind up" .)