Environmentalist gets one cool treatment (9)
I believe the answer is:
ecologist
'environmentalist' is the definition.
(I've seen this before)
'gets one cool treatment' is the wordplay.
'one' becomes 'i' (Roman numeral).
'treatment' indicates anagramming the letters (letters treated or processed differently).
'gets'+'i'+'cool'='getsicool'
'getsicool' is an anagram of 'ECOLOGIST'.
(Other definitions for ecologist that I've seen before include "Company's possessions" , "One studying the relation between organisms and their environment" , "Student of living things and their environment" , "See scientist" , "One concerned with the environment" .)