Green film about operating organ (9)
I believe the answer is:
economist
'organ' is the definition.
(organ can mean a magazine eg The Economist)
'green film about operating' is the wordplay.
'green' becomes 'eco' (abbreviation for ecological).
'film' becomes 'mist' (I've seen this in other clues**).
'about' means one lot of letters goes inside another.
'operating' becomes 'on' (I've seen this in other clues).
'eco'+'mist'='ecomist'
'ecomist' enclosing 'on' is 'ECONOMIST'.
(Other definitions for economist that I've seen before include "Expert on finances" , "Money supply expert" , "publication" , "Expert in the management of a country's material resources" , "One versed in the management of a country's material resources" .)