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 "Grow worse", "Become slowly worse", "Get worse", "Gradually worsen", "Become steadily worse", "Become progressively worse" and "Worsen".)