Detective's anger about old man getting run down (9)
I believe the answer is:
disparage
'run down' is the definition.
(I've seen this before)
'detective's anger about old man' is the wordplay.
'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' (both can mean father).
'rage' put after 'pa' is 'parage'.
'dis'+'parage'='DISPARAGE'
'getting' acts as a link.
(Other definitions for disparage that I've seen before include "Belittled" , "Run down" , "I grasped a change to belittle" , "Slight" , "Scorn" .)