Don't accept deterioration (7)
I believe the answer is:
decline
'deterioration' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for decline that I've seen before include "Turn down; deteriorate" , "Start to fail" , "Refuse - to take a drop" , "The classics student must often" , "Fade" .)