Change in speed I thought (4)
I believe the answer is:
edit
'change' is the definition.
(edit can mean to change or alter)
'in speed i thought' is the wordplay.
'in' indicates the answer is hidden within the clue.
'EDIT' is hidden within 'speed i thought'.
(Other definitions for edit that I've seen before include "Manage a newspaper" , "Alter text" , "Check text for publication" , "What our boss does" , "Be in charge of (newspaper)" .)