Is fat, certainly, though not exactly butter (6)

I believe the answer is:

bunter

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But I can't explain it at all! Could you help me understand?

(Other definitions for bunter that I've seen before include "Unending time" , "Billy -, fict. schoolboy" , "Billy ..., the Owl of the Remove at Greyfriars" , "Billy ......, iconic English fat boy" .)

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