Birthday-to-birthday span (4)
I believe the answer is:
year
'birthday-to-birthday span' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for year that I've seen before include "Twelvemonth" , "See 33 Across" , "Four seasons" , "Jan to Dec" , "Period of just over 52 weeks" .)