Discourage one old judge in decline (11)
I believe the answer is:
Here is my best explanation:
'decline' is the definition.
(deteriorating is a kind of declining)
'discourage one old judge' is the subsidiary indication.
'discourage' becomes 'deter' ('deter' can be a synonym of 'discourage').
'one' becomes 'i' (roman numeral).
'old' becomes 'o'.
'judge' becomes 'rate' (rating is a kind of judging**).
'in' is the link.
(Other definitions for deteriorate that I've seen before include "Fail", "Worsen", "decline", "Slip", "Get worse", "Gradually worsen", "Become slowly worse", "Go downhill", "Become progressively worse" and "Become steadily worse".)