Left, having finished really early (4)
I believe the answer is:
quit
'left' is the definition.
(I've seen this in other clues)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for quit that I've seen before include "Leave, depart" , "Desist" , "Leave or abandon" , "Abandon, depart" , "Leave - resign" .)