Going down? About time - it's disgusting (8)
I believe the answer is:
stinking
'disgusting' is the definition.
(thesaurus)
'going down? about time' is the wordplay.
'going down?' becomes 'sinking' ('sink' can be a synonym of 'go down').
'about' means one lot of letters goes inside another.
'time' becomes 't' (abbreviation).
'sinking' placed around 't' is 'STINKING'.
'it's' acts as a link.
(Other definitions for stinking that I've seen before include "rotten!" , "obnoxious" , "Foul-smelling" , "Rank" , "smelly" .)