2004, for one (4,4)
I believe the answer is:
leap year
'2004 for one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for leap year that I've seen before include "Three hundred and sixty six-day period" , "March delayed now" , "Three hundred and sixty six - day period" , "2008? - with 24d" , "When the lady may traditionally propose" .)