A part once it's revised (7)
I believe the answer is:
section
'a part' is the definition.
(I know that part can be written as section)
'once it's revised' is the wordplay.
'revised' indicates anagramming the letters.
'once'+'its'='onceits'
'onceits' anagrammed gives 'SECTION'.
(Other definitions for section that I've seen before include "Department, division" , "Subdivision of a newspaper" , "Part, subdivision" , "branch" , "unit" .)