Go green, changing policy finally (6)
I believe the answer is:
energy
'go' is the definition.
(thesaurus)
'green changing policy finally' is the wordplay.
'changing' indicates an anagram.
'finally' indicates one should take the final letters.
The last letter of 'policy' is 'y'.
'green' anagrammed gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Fuel" , "Verve" , "Dynamism" , "Zeal" , "life" .)