Leap year's first full season (6)
I believe the answer is:
spring
I believe this clue is a double definition.
'leap' is the first definition.
(leap can mean to spring or jump)
'year's first full season' is the second definition.
(first full season of a calendar year)
(Other definitions for spring that I've seen before include "See 11" , "Season of growth" , "Well - bound" , "beginning" , "Shoot forth" .)