Bound to be referring to year of particular type (4)
I believe the answer is:
leap
I believe this clue is a double definition.
'bound' is the first definition.
(I know that bound can be written as leap)
'referring to year of particular type' is the second definition.
(leap year)
'to be' is the link.
(Other definitions for leap that I've seen before include "Sort of year" , "'Look before you . . . .'" , "identify extra day?" , "Jump across" , "Sudden surge" .)