Sick of being fattened (3,2)

I believe the answer is:

fed up

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'sick of' is the definition.
(synonyms)

I can't explain the remainder of the clue.

(Other definitions for fed up that I've seen before include "like infant in high chair?" , "Bored, browned off" , "Annoyed, bored" , "Hacked off" , "Disaffected" .)

I've seen this clue in the Evening Standard.

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