Marvellous young environmentalist's moving article (5)

I believe the answer is:

great

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'marvellous' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for great that I've seen before include "Considerably above average" , "'Goodness gracious -- -- -- --' (pop song)" , "Dickens novel about Pip" , "Excellent (colloq.)" , "Of considerable size" .)

I've seen this clue in The Guardian.

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