Project involving a worry about Pat’s constituent? (9)

I believe the answer is:

butterfat

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'project involving a worry' is the definition.
I can't judge whether this definition defines the answer.

'pat's constituent?' is the wordplay.
'pat' becomes 'butter' (pat can mean a lump of butter).
'constituent?' becomes 'fat' (I am not sure about this - if you are sure you should believe this answer much more).
'butter'+'fat'='BUTTERFAT'

'about' acts as a link.

Can you help me to learn more?

(Other definitions for butterfat that I've seen before include "Dairy product" , "Ingredient of milk" .)

I've seen this clue in The Times.

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