Deteriorates yet visits toilet (4,2,3)
I believe the answer is:
goes to pot
'deteriorates' is the definition.
(I've seen this before)
'visits toilet' is the wordplay.
'visits' becomes 'goes to' (I've seen this in another clue).
'toilet' becomes 'pot' (synonyms).
'goesto'+'pot'='GOES TO POT'
'yet' is the link.
(Other definitions for goes to pot that I've seen before include "Becomes ruined" , "Approaches the snooker table" , "Become ruined" , "loses form" , "cracks up" .)