Set against foreign volunteers returning to Spain (8)
I believe the answer is:
alienate
'set against' is the definition.
(thesaurus)
'foreign volunteers returning to spain' is the wordplay.
'foreign' becomes 'alien' ('alien' can be a synonym of 'foreign').
'volunteers' becomes 'ta' (abbreviation for Territorial Army).
'returning' shows that the letters should be reversed in order.
'to spain' becomes 'e' (abbreviation for Espana).
'ta' written backwards gives 'at'.
'alien'+'at'+'e'='ALIENATE'
(Other definitions for alienate that I've seen before include "cut off" , "Cause (someone) to feel isolated" , "Turn away from" , "Set against" , "Distance" .)