USA cred changed campaign (7)
I believe the answer is:
crusade
'campaign' is the definition.
(I know that campaign can be written as crusade)
'usa cred changed' is the wordplay.
'changed' indicates an anagram.
'usa'+'cred'='usacred'
'usacred' is an anagram of 'CRUSADE'.
(Other definitions for crusade that I've seen before include "Medieval Christian military expedition to capture the Holy Land" , "Holy war" , "Vigorous campaign" , "Drive" , "Religious war" .)