Good criteria for treatment of the old (9)
I believe the answer is:
geriatric
'the old' is the definition.
(I've seen this before)
'good criteria for treatment' is the wordplay.
'good' becomes 'g' (abbreviation).
'for treatment' indicates an anagram (I've seen this in other clues).
'criteria' anagrammed gives 'eriatric'.
'g'+'eriatric'='GERIATRIC'
'of' acts as a link.
(Other definitions for geriatric that I've seen before include "Relating to old people" , "Oldie" , "treatment of the elderly" , "past it" .)