Having almost left, Charlie enters to stir things up (6)
I believe the answer is:
excite
'up' is the definition.
The definition and answer can be both related to change as well as being verbs in their base form.
Maybe they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
(Other definitions for excite that I've seen before include "Thrill" , "Arouse, make eager" , "Cause (someone) to feel very enthusiastic" , "cause a stir" , "Inflame or stimulate" .)