Corroding (7)
(Other definitions for rotting that I've seen before include "Going bad" , "corruption?" , "Putrid" , "Decaying" , "Sultanate in NW Borneo" .)
'corroding' is the definition.
(I know that rusting is a type of corroding)
This is all the clue.
(Another definition for rusting that I've seen is " Corroding".)