Decline to take the dog back, but have to (2,4)
I believe the answer is:
go down
'decline' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for go down that I've seen before include "Fall" , "Descend - be relegated" , "Subside" , "Decline" , "leave university" .)