Crossword Genius

Are they silly enough to get fatally fat? (5)

Ross

I believe the answer is:

geese

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'are they' is the definition.
The definition suggests a plural noun which matches the answer.

I can't explain the rest of the clue.

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(Other definitions for geese that I've seen before include "Large farm birds" , "Large water-birds with webbed feet and broad bills" , "Protectors of city" , "Birds that fly in a skein" , "They come in gaggles and skeins" .)

I've seen this clue in the Evening Standard.
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