Crossword Genius

Big Mike trapped by scary alien space travellers (6,4)

Ross

I believe the answer is:

cosmic rays

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'space travellers' is the definition.
The definition suggests a plural noun which matches the answer.

'big mike trapped by scary alien' is the wordplay.
'big' becomes 'os'.
'mike' becomes 'mic' (I've seen this in other clues).
'trapped by' means one lot of letters goes inside another.
'alien' is an anagram indicator.
'os'+'mic'='osmic'
'scary' is an anagram of 'crays'.
'osmic' put inside 'crays' is 'COSMIC RAYS'.

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I've seen this clue in The Guardian.
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