Last month city turned greener, to announce start of campaign (7,3)
I believe the answer is:
declare war
'announce start of campaign' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'last month city turned greener' is the wordplay.
'last month' becomes 'dec' (short for December, last month of the year).
'city' becomes 'LA' (Los Angeles).
'turned' is a reversal indicator.
'greener' becomes 'rawer' ('raw' can be a synonym of 'green').
'rawer' written backwards gives 'rewar'.
'dec'+'la'+'rewar'='DECLARE WAR'
'to' is the link.
Can you help me to learn more?
(Another definition for declare war that I've seen is " state intention to fight".)