Film with Newton-John being fat (6)

I believe the answer is:

grease

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

I don't understand how the rest of the clue works.

(Other definitions for grease that I've seen before include "Lubricate - musical" , "Musical starring John Travolta and Olivia Newton-John" , "Rock musical" , "bribe" , "A thick oily substance" .)

I've seen this clue in The Sun.

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