Awful rat, Spain supporter (8)
I believe the answer is:
partisan
'supporter' is the definition.
(I know that partisan is a type of supporter)
'awful rat spain' is the wordplay.
'awful' indicates anagramming the letters.
'rat'+'spain'='ratspain'
'ratspain' with letters rearranged gives 'PARTISAN'.
(Other definitions for partisan that I've seen before include "Prejudiced in favour of a particular cause" , "Traps in a prejudiced way" , "Arts pain (anag.)" , "Strongly supporting one side" , "Guerrilla" .)