Legal document fool took across river (5,4)
I believe the answer is:
title deed
'legal document' is the definition.
(title deed is a kind of legal document)
'fool took across river' is the wordplay.
'fool' becomes 'tit' (I've seen this in other clues**).
'took' becomes 'led' ('lead' can be a synonym of 'take').
'across' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'led' going around 'dee' is 'ledeed'.
'tit'+'ledeed'='TITLE-DEED'
(Other definitions for title deed that I've seen before include "Document establishing legal right to property" , "Document proving ownership" , "Deleted it (anag.)" , "proper document" , "Having proof of ownership" .)