Advantage gained by beginning early (4,5)
I believe the answer is:
head start
'advantage gained by beginning early' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for head start that I've seen before include "Advantage gained by beginning early" , "A good beginning using one's loaf" , "'The advantage of beginning ahead of others (4,5)'" , "An early advantage for the star ad" , "Substantial initial advantage" .)