“Alien to spoil it after turning up” - a Nationalist (7)
I believe the answer is:
martian
'alien' is the definition.
(I've seen this before)
'spoil it after turning up a nationalist' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'after turning up' is a reversal indicator.
'nationalist' becomes 'n' (abbreviation in politics).
'it' written backwards gives 'ti'.
'mar'+'ti'+'a'+'n'='MARTIAN'
'to' is the link.
(Other definitions for martian that I've seen before include "One living on red planet" , "Invader in The War of the Worlds (H. G. Wells)" , "Sci-fi alien" , "Relating to a particular planet" , "Being from another planet" .)