A firm order to dispose of rust (7)
I believe the answer is:
corrode
'rust' is the definition.
('corrode' can be a synonym of 'rust')
'a firm order to dispose' is the wordplay.
'a firm' becomes 'co' (a firm is a company).
'to dispose' indicates an anagram (I've seen 'disposed' mean this).
'order' with letters rearranged gives 'rrode'.
'co'+'rrode'='CORRODE'
'of' is the link.
(Other definitions for corrode that I've seen before include "Growth in hazel buds" , "Oxidise" , "Eat away, rust" , "Gradually weaken (metal)" , "Oxidize" .)