Once in a __, almost never (4,4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
blue moon
(Other definitions for blue moon that I've seen before include "Very rare occurrence" , "Rodgers and Hart song" , "In which once is very seldom" , "Once in one is a rare event" , "Very rarely, if once in one of these" .)