Quotation from an old treatise (7)
I believe the answer is:
extract
'quotation' is the definition.
(an extract is quotation or selection from a text)
'an old treatise' is the wordplay.
'an old' becomes 'ex'.
'treatise' becomes 'tract' (tract is a kind of treatise).
'ex'+'tract'='EXTRACT'
'from' acts as a link.
(Other definitions for extract that I've seen before include "Get out" , "essence" , "Short piece" , "Piece taken from a longer work" , "Part" .)