Tents pitched on major road by extremists in Green lobby (8)
I believe the answer is:
campaign
'lobby' is the definition.
(I've seen this before)
'tents pitched on major road by extremists in green' is the wordplay.
'tents pitched' becomes 'camp' (a camp site).
'on' says to put letters next to each other.
'major road' becomes 'AI' (resembles A1, a major road).
'by' says to put letters next to each other.
'extremists in' suggests removing the centre.
'green' with its middle taken out is 'gn'.
'camp'+'ai'+'gn'='CAMPAIGN'
(Other definitions for campaign that I've seen before include "electioneering" , "battle" , "Series of actions toward a particular end" , "Series of military operations intended to achieve a goal" , "(E.g. military, political) project" .)