One under ten confused by legal deed (9)
I believe the answer is:
indenture
'legal deed' is the definition.
(type of legal document)
'one under ten confused' is the wordplay.
'one' becomes 'I' (Roman numeral).
'confused' is an anagram indicator.
'under'+'ten'='underten'
'underten' with letters rearranged gives 'ndenture'.
'i'+'ndenture'='INDENTURE'
'by' is the link.
(Other definitions for indenture that I've seen before include "Agreement binding apprentice to a master" , "Sealed agreement" , "served by a novice?" , "Bind to service" , "contract" .)