366 days (4,4)
I believe the answer is:
leap year
'366 days' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for leap year that I've seen before include "Period with an intercalary day" , "when summer Olympics are held?" , "Three hundred and sixty six-day period" , "It contains an extra day" , "366, not 365 days" .)