Crossword Genius

Declare nation fat (6)

Ross

I believe the answer is:

grease

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'fat' is the definition.
(animal fat for cooking)

I cannot understand how the rest of the clue works.

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(Other definitions for grease that I've seen before include "'Thick, fatty oil (6)'" , "Fatty lubricant" , "Musical - lubricant" , "Fat, film" , "-- gun; -- monkey" .)

I've seen this clue in the Sydney Morning Herald.
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