Declare nation fat (6)
I believe the answer is:
grease
'fat' is the definition.
(animal fat for cooking)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for grease that I've seen before include "'Thick, fatty oil (6)'" , "Fatty lubricant" , "Musical - lubricant" , "Fat, film" , "-- gun; -- monkey" .)