Discourage one old judge in decline (11)
I believe the answer is:
deteriorate
'decline' is the definition.
(deteriorating is a kind of declining)
'discourage one old judge' is the wordplay.
'discourage' becomes 'deter' ('deter' can be a synonym of 'discourage').
'one' becomes 'i' (Roman numeral).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'judge' becomes 'rate' (rating is a kind of judging).
'deter'+'i'+'o'+'rate'='DETERIORATE'
'in' acts as a link.
(Other definitions for deteriorate that I've seen before include "Fail" , "Gradually worsen" , "Get worse" , "Slip" , "Become slowly worse" .)