Essentially toxic delta area turning into rust (9)
I believe the answer is:
oxidation
'rust' is the definition.
(thesaurus)
'essentially toxic delta area turning into' is the wordplay.
'essentially' indicates the central letters.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'area' becomes 'a' (maths abbreviation).
'turning' is an anagram indicator.
The middle of 'toxic' is 'oxi'.
'into' with letters rearranged gives 'tion'.
'oxi'+'d'+'a'+'tion'='OXIDATION'
(Other definitions for oxidation that I've seen before include "Rusting process" , "rust" , "instance of becoming dull?" , "Eg, rusting" .)