Passed on document about complicated case (8)
I believe the answer is:
deceased
'passed on' is the definition.
(thesaurus)
'document about complicated case' is the wordplay.
'document' becomes 'deed' (deed is a kind of document).
'about' is an insertion indicator.
'complicated' is an anagram indicator (a complex or confusing ordering of the letters).
'case' anagrammed gives 'ceas'.
'deed' enclosing 'ceas' is 'DECEASED'.
(Other definitions for deceased that I've seen before include "Recently dead" , "No longer living" , "Late" , "Defunct" , "Pushing up the daisies" .)