Car used somehow for a campaign (7)
I believe the answer is:
crusade
'for a campaign' is the definition.
('crusade' can be a synonym of 'campaign')
'car used somehow' is the wordplay.
'somehow' indicates anagramming the letters.
'car'+'used'='carused'
'carused' is an anagram of 'CRUSADE'.
(Other definitions for crusade that I've seen before include "Advertising campaign" , "Religious war" , "Action by Christians to recapture the Holy Land" , "Vigorous campaign" , "Medieval Christian military expedition to capture the Holy Land" .)