Happen again (and again?) (5)
I believe the answer is:
recur
'happen again and again?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for recur that I've seen before include "come back to one's mind" , "Happen repeatedly" , "Return to mind" , "Happen again (5)" , "expected to return" .)