Change poor diet (4)
I believe the answer is:
edit
'change' is the definition.
(edit can mean to change or alter)
'poor diet' is the wordplay.
'poor' indicates anagramming the letters.
'diet' is an anagram of 'EDIT'.
(Other definitions for edit that I've seen before include "Alter text" , "Supervise the publication of" , "work on newspaper?" , "be a bookmaker" , "Literally, arrange" .)