Receding icecaps partially observed from area over 100km above sea level (5)

I believe the answer is:

space

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'level' is the definition.
(thesaurus)

'receding icecaps partially observed from area over 100km above sea' is the wordplay.
'SPACE' is hidden backwards within the letters.
However, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for space that I've seen before include "A particular area for paces" , "Unfilled region" , "Cosmos" , "Gap between words" , ""The final frontier"" .)

I've seen this clue in The Irish Times.

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