Paper -- originally it's a tree (8)
I believe the answer is:
treatise
'paper' is the definition.
(I've seen this before)
'originally it's a tree' is the wordplay.
'originally' indicates an anagram.
'its'+'a'+'tree'='itsatree'
'itsatree' with letters rearranged gives 'TREATISE'.
(Other definitions for treatise that I've seen before include "Long, detailed essay" , "A written work about treaties" , "Formal written work dealing with a particular subject" , "Written exposition of an argument" , "Iterates in dissertation" .)