He's so late starting! (3)
I believe the answer is:
sol
'he's' is the definition.
'so late starting' is the wordplay.
'starting' indicates taking the first letters.
The initial letter of 'late' is 'l'.
'so'+'l'='SOL'
(Other definitions for sol that I've seen before include "Note, old french coin" , "Note - colloid - the Sun" , "Roman counterpart of Helios" , "A Latin sun" , "The sun in 12 Across" .)