Legal document settled a tiff David ultimately lost (9)
I believe the answer is:
affidavit
'legal document' is the definition.
(affidavit is a kind of legal document)
'settled a tiff david ultimately lost' is the wordplay.
'settled' indicates anagramming the letters.
'ultimately lost' means to remove the last letter.
'a'+'tiff'+'david'='atiffdavid'
'atiffdavid' with its final letter taken away is 'atiffdavi'.
'atiffdavi' anagrammed gives 'AFFIDAVIT'.
(Other definitions for affidavit that I've seen before include "Written evidence" , "Declaration before notary" , "Sworn court statement" , "legal document" , "Sworn statement for legal purposes" .)