Boy starting school, getting ahead (3)
I believe the answer is:
son
'boy' is the definition.
(I know that boy can be written as Son)
'starting school getting ahead' is the wordplay.
'starting' says to take the initial letters.
'getting' says to put letters next to each other.
'ahead' becomes 'on' (synonyms).
The first letter of 'school' is 's'.
's'+'on'='SON'
(Other definitions for son that I've seen before include "Male offspring" , "Heir" , "Express paper" , "Male descendant" , "A relative" .)