Change in speed, I think (4)
I believe the answer is:
edit
'change' is the definition.
(editing is a kind of changing)
'in speed i think' is the wordplay.
'in' says the answer is hidden in the clue.
'EDIT' is hidden within 'speed i think'.
(Other definitions for edit that I've seen before include "Tide (anag.)" , "Amend and correct" , "This is an act of cleaning" , "be a bookmaker" , "Fix" .)