Green militant with energy to bully a bishop downing port (3-7)

'green militant' is the definition.
(someone fighting for green causes)

'energy to bully a bishop downing port' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mcr).
'to bully' becomes 'cow' (cow can mean to bully or oppress).
'bishop' becomes 'rr' (abbreviation for Right Reverend).
'downing' means one lot of letters goes inside another (to down food or drink is to swallow it quickly).
'port' becomes 'rio' (Rio de Janeiro is a port).
'rr' going around 'rio' is 'rrior'.
'e'+'cow'+'a'+'rrior'='ECO-WARRIOR'

'with' acts as a link.