Vigorous campaign (7)
I believe the answer is:
crusade
'vigorous campaign' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for crusade that I've seen before include "Used car involved in campaign for a cause" , "Medieval Christian military expedition to capture the Holy Land" , "Vigorous campaign" , "Action by Christians to recapture the Holy Land" , "Religious war" .)