Main art installation is alien (7)

I believe the answer is:

martian

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'is alien' is the definition.
(a lifeform from Mars)

'main art installation' is the wordplay.
'main art' can be anagrammed to 'MARTIAN'.
However, I'm not clear how the anagram is indicated.

Can you help me to learn more?

(Other definitions for martian that I've seen before include "Green man from the Red Planet" , "Sci-fi alien" , "Sci-fi character" , "Relating to a particular planet" , "One living on red planet" .)

I've seen this clue in The Mirror.

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