Against clergyman returning to South America (6)
I believe the answer is:
versus
'against' is the definition.
(thesaurus)
'clergyman returning to south america' is the wordplay.
'clergyman' becomes 'rev' (I've seen this before).
'returning' says the letters should be written in reverse.
'to south' becomes 's' ('S' can be a synonym of 'south').
'america' becomes 'us' (abbreviation for the United States).
'rev' backwards is 'ver'.
'ver'+'s'+'us'='VERSUS'
(Other definitions for versus that I've seen before include "Against in games" , "In opposition to" , "v. (sport)" , "Opposed to" , "Against (opponent)" .)