It's one in four (4,4)
I believe the answer is:
leap year
'it's one in four' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for leap year that I've seen before include "Two thousand and four the next one" , "When the lady may traditionally propose" , "when summer Olympics are held?" , "It has one extra day" , "March delayed now" .)