Overfed gym rat starts to annoy (5)

I believe the answer is:

grate

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'to annoy' is the definition.
(I know that grate is a more specific form of annoy)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for grate that I've seen before include "Grind, rasp" , "part of fireplace" , "Reduce to small shreds" , "Grind (teeth)" , "Make a harsh rasping sound" .)

I've seen this clue in The Daily Express.

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