An essential part of the WI? (3)

I believe the answer is:

jam

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'an essential part' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe there's an association between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for jam that I've seen before include "problem" , "Troublesome situation" , "Fruit preserve" , "Block signals" , "Preserve; pack tightly" .)

I've seen this clue in The Times.

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