Purchasing company, maintain noted new process of removing harmful substances (15)
I believe the answer is:
decontamination
'process of removing harmful substances' is the definition.
(removing contaminated substances)
'purchasing company maintain noted new' is the wordplay.
'purchasing' is an insertion indicator (some letters take possession of others).
'company' becomes 'co' (abbreviation).
'new' indicates anagramming the letters.
'maintain'+'noted'='maintainnoted'
'maintainnoted' is an anagram of 'dentamination'.
'co' put into 'dentamination' is 'DECONTAMINATION'.
(Other definitions for decontamination that I've seen before include "debugging?" , "act of cleansing" , "The removal of dangerous substances" , "Process of removing dangerous substances" .)