Stop taking part in open days (3)
I believe the answer is:
end
'stop' is the definition.
(I know that stop can be written as end)
'part in open days' is the wordplay.
'part in' says the answer is hidden in the clue.
'END' can be found hidden inside 'open days'.
'taking' acts as a 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" .)