Vigorous campaign (7)
I believe the answer is:
crusade
'vigorous campaign' is the definition.
(I've seen this in another clue)
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" .)