Substance producing rust in west, reportedly (7)
I believe the answer is:
oxidant
'substance producing rust' is the definition.
(an oxidant causes rust)
'west reportedly' is the wordplay.
'west' becomes 'occident' (as opposed to the orient, the east).
'reportedly' shows a homophone (sound like).
'occident' is a homophone of 'OXIDANT'.
'in' acts as a link.
(Other definitions for oxidant that I've seen before include "Something causing rust, for instance" , "chemical agent" , "agent causing deterioration" , "Substance such as hydrogen peroxide" , "Such as hydrogen peroxide or ozone" .)