Left section to fill in document (8)
I believe the answer is:
departed
'left' is the definition.
(departing is a kind of leaving)
'section to fill in document' is the wordplay.
'section' becomes 'part' ('part' can be a synonym of 'section').
'to fill' means one lot of letters goes inside another.
'in document' becomes 'deed' (deed is a kind of document. I am not sure about the 'in' bit.**).
'part' inserted inside 'deed' is 'DEPARTED'.
Can you help me to learn more?
(Other definitions for departed that I've seen before include "Left, went away" , "completely breathless" , "Dead; set off" , "Left -- late" , "Gone, maybe dead" .)