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Mushroom cloud ultimately can't be reduced? (8)

Ross

I believe the answer is:

enlarged

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'reduced?' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for enlarged that I've seen before include "Made bigger, say photo" , "Increased, augmented" , "Bloated" , "Expanded, amplified" , "Made greater" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).
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