Toilet with blue stall (3,4)
I believe the answer is:
bog down
'stall' is the definition.
The answer and definition can be both related to change as well as being verbs in their base form.
Perhaps there's a link between them I don't understand?
'toilet with blue' is the wordplay.
'toilet' becomes 'bog' (informal term for toilet).
'with' says to put letters next to each other.
'blue' becomes 'down' (both can mean sad).
'bog'+'down'='BOG-DOWN'
Can you help me to learn more?
(Other definitions for bog down that I've seen before include "Impede" , "Encumber, hinder" , "Encumber (with work, etc)" .)