Crossword Genius

One choosing to be more obese? (7)

Ross

I believe the answer is:

plumper

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'one choosing' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for plumper that I've seen before include "more stout" , "One choosing" , "Fuller and rounder" .)

I've seen this clue in The Independent.
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