A part once it's recast (7)
I believe the answer is:
section
'a part' is the definition.
(section is a kind of part)
'once it's recast' is the wordplay.
'recast' indicates anagramming the letters.
'once'+'its'='onceits'
'onceits' is an anagram of 'SECTION'.
(Other definitions for section that I've seen before include "Segment" , "Notices (anag)" , "Divide; part" , "Element" , "branch" .)