Earth Day month (5)
I believe the answer is:
april
'earth day month' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for april that I've seen before include "First of Herrick's four sweet months" , "whose first date is foolish?" , "The month with All Fools' Day" , "Month of showers and of the Queen's birthday" , "'All in an ... evening' (Katharine Tynan)" .)