Sign of inactivity
I believe the answer is:
'sign of inactivity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rust that I've seen before include "Oxidised iron" , "Red or brown oxide coating on iron or steel" , "Reddish-brown coating caused by oxidation" , "Reddish coating formed on iron through 11 across" , "Oxidisation" .)