Environmentalist to sponsor bill in Washington (9)
I believe the answer is:
greenback
'bill in washington' is the definition.
(informal term for one dollar bill)
'environmentalist to sponsor' is the wordplay.
'environmentalist' becomes 'green' (I've seen this before).
'to sponsor' becomes 'back'.
'green'+'back'='GREENBACK'
(Other definitions for greenback that I've seen before include "Dollar bill" , "money" .)