Old cross an idiot forged getting rusty? (9)
I believe the answer is:
oxidation
'getting rusty?' is the definition.
(process of getting rusty or oxidised)
'old cross an idiot forged' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'cross' becomes 'x' (X is cross-shaped).
'forged' is an anagram indicator.
'an'+'idiot'='anidiot'
'anidiot' is an anagram of 'idation'.
'o'+'x'+'idation'='OXIDATION'
(Other definitions for oxidation that I've seen before include "instance of becoming dull?" , "rust" , "Rusting process" , "Eg, rusting" .)