Metal that rusts (4)
I believe the answer is:
iron
'metal that rusts' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iron that I've seen before include "medicinal preparation for the anaemic?" , "resolve" , "Metal (that may be wrought)" , "Metal obtained from haematite" , "Flatten; element" .)