Tireless campaign (7)
I believe the answer is:
crusade
'tireless 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 "Religious war" , "Advertising campaign" , "Action by Christians to recapture the Holy Land" , "Drive" , "Medieval Christian military expedition to capture the Holy Land" .)