Rewrite part of poem, 'Endymion' (5)
I believe the answer is:
emend
'rewrite' is the definition.
(thesaurus)
'part of poem endymion' is the wordplay.
'part of' says the answer is hidden in the clue.
'EMEND' is hidden in the letters of 'poem endymion'.
(Other definitions for emend that I've seen before include "Edit; make alterations to a draft" , "change" , "Correct/revise (a text)" , "make an improvement" , "Make improvements or corrections" .)