Manuscript's first page rewritten about one hoarder (6)
I believe the answer is:
magpie
'hoarder' is the definition.
(synonyms)
'manuscript's first page rewritten about one' is the wordplay.
'manuscript's first' becomes 'm' (1st letter of 'manuscript').
'rewritten' is an anagram indicator.
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'page' anagrammed gives 'agpe'.
'agpe' enclosing 'i' is 'agpie'.
'm'+'agpie'='MAGPIE'
(Other definitions for magpie that I've seen before include "Ominous bird if seen alone" , "Long-tailed crow" , "1 [UNLUCKY FOR SOME]" , "Common bird" , "Bird of the crow family" .)