Left some enthralled by legal document (8)
I believe the answer is:
departed
'left' is the definition.
(departing is a kind of leaving)
'some enthralled by legal document' is the wordplay.
'some' becomes 'part'.
'enthralled by' is an insertion indicator ('enthral' can mean 'capture').
'legal document' becomes 'deed' (deed is a kind of legal document).
'part' put into 'deed' is 'DEPARTED'.
(Other definitions for departed that I've seen before include "Went - dead" , "Dead; set off" , "Left -- late" , "Gone, maybe dead" , "completely breathless" .)