Imaginary invader in a train wreck (7)

I believe the answer is:

martian

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'imaginary' is the definition.
Although both the answer and definition are adjectives, I can't see how one could define the other.

'invader in a train wreck' is the wordplay.
'invader in' becomes 'ma' (I can't justify this - if you can you should believe this answer much more).
'wreck' indicates an anagram.
'train' is an anagram of 'rtian'.
'ma'+'rtian'='MARTIAN'

Can you help me to learn more?

(Other definitions for martian that I've seen before include "seeing little green man?" , "One of H. G. Wells's invaders" , "Sci-fi character" , "One living on red planet" , "Relating to one planet" .)

I've seen this clue in the Evening Standard.

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