South America invested in oil campaign (7)
I believe the answer is:
crusade
'campaign' is the definition.
('crusade' can be a synonym of 'campaign')
'south america invested in oil' is the wordplay.
'south' becomes 's' ('S' can be a synonym of 'south').
'america' becomes 'a' (common abbreviation - e.g. in organisation names).
'invested in' indicates putting letters inside.
'oil' becomes 'crude' (crude is a kind of oil).
's'+'a'='sa'
'sa' inserted within 'crude' is 'CRUSADE'.
(Other definitions for crusade that I've seen before include "Action by Christians to recapture the Holy Land" , "Drive" , "Used car involved in campaign for a cause" , "Holy war" , "Medieval Christian military expedition to capture the Holy Land" .)