Cook, large inside, getting more slender (7)

I believe the answer is:

slimmer

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'more slender' is the definition.
('slim' can be a synonym of 'slender')

I can't explain the rest of the clue.

(Other definitions for slimmer that I've seen before include "More slenderly built" , "One losing weight" , "person who's trying to lose weight" , "More slender" , "I aim to get rid of some" .)

I've seen this clue in The Telegraph.

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