Spoil it turning to an extra-terrestrial (7)

I believe the answer is:

martian

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'extra-terrestrial' is the definition.
Both the definition and answer are adjectives.
Maybe you can see an association between them that I don't see?

'spoil it turning to an' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'turning to' indicates an anagram (the letters can turn into other words).
'it' anagrammed gives 'ti'.
'mar'+'ti'+'an'='MARTIAN'

Can you help me to learn more?

(Other definitions for martian that I've seen before include "Sci-fi character" , "Being from another planet" , "Green man from the Red Planet" , "seeing little green man?" , "One living on red planet" .)

I've seen this clue in the Sydney Morning Herald.

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