Changes are green first these days (6,3)
I believe the answer is:
common era
'these days' is the definition.
(the current era)
'changes are green first' is the wordplay.
'changes' indicates anagramming the letters.
'green' becomes 'common' (synonyms).
'first' says to put letters next to each other.
'are' is an anagram of 'era'.
'era' put after 'common' is 'COMMON ERA'.
(Other definitions for common era that I've seen before include "these days" , "In Christian times" , "this time" .)