Guerrilla with role in repressing South America (8)
I believe the answer is:
partisan
'guerrilla' is the definition.
(I've seen this before)
'role in repressing south america' is the wordplay.
'role' becomes 'part' ('part' can be a synonym of 'role').
'repressing' means one lot of letters goes inside another.
'south' becomes 's' ('S' can be a synonym of 'south').
'america' becomes 'a' (common abbreviation - e.g. in organisation names).
's'+'a'='sa'
'in' placed around 'sa' is 'isan'.
'part'+'isan'='PARTISAN'
'with' is the link.
(Other definitions for partisan that I've seen before include "One-sided" , "Unquestioning supporter -- aspirant (anag)" , "Devotee of a cause" , "Prejudiced towards one side" , "Enthusiastic supporter" .)