Detective's anger about old man getting run down (9)
I believe the answer is:
Here is my best explanation:
'run down' is the definition.
'detective's anger about old man' is the subsidiary indication.
'detective's' becomes 'dis' (Detective Inspector).
'anger' becomes 'rage' (rage is a kind of anger**).
'about' means one lot of letters go next to another.
'old man' becomes 'pa' ('pa' can be a synonym of 'old man').
'rage' put after 'pa' is 'parage'.
'getting' is the link.
(Other definitions for disparage that I've seen before include "I grasped a change to belittle", "Belittle", "Belittled", "Scorn", "Run down", "Slight" and "run down".)