Big old boy feasted regularly (5)

I believe the answer is:

obese

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'big' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for obese that I've seen before include "Extremely fat" , "Unhealthily large" , "Morbidly fat" , "Excessively fat" , "having generous corporation" .)

I've seen this clue in The Sun.

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