Once in one is a rare event (4,4)
I believe the answer is:
blue moon
'once in one is a rare event' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for blue moon that I've seen before include "'If it happens once in it, it's hardly ever (4,4)'" , "If something happens once in it, it's not very often" , "Once in one is a rare event" , "In which once is very seldom" , "Coloured lunar phenomenon, happens rarely" .)