Out-and-out nationalist ultimately supporting Scottish isle (6)
I believe the answer is:
arrant
'out-and-out' is the definition.
('arrant' can be a synonym of 'out-and-out')
'nationalist ultimately supporting scottish isle' is the wordplay.
'ultimately' indicates one should take the final letters.
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'scottish isle' becomes 'arran'.
The final letter of 'nationalist' is 't'.
't' put after 'arran' is 'ARRANT'.
(Other definitions for arrant that I've seen before include "Extreme or flagrant, perhaps nonsense" , "Utter; downright" , "Utter, complete" , "Rat ran badly and became unmitigated" , "Downright - notorious" .)