Come again; happen once more (5)
I believe the answer is:
recur
I believe this clue is a double definition.
'come again' is the first definition.
(I've seen this before)
'happen once more' is the second definition.
(I've seen this before)
(Other definitions for recur that I've seen before include "Happen again (5)" , "Happen again or repetitively" , "Perseverate" , "Come round again" , "expected to return" .)