Something you didn’t polish off on your plate (7)

I believe the answer is:

tarnish

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'plate' is the definition.
The answer and definition can be both to do with contact as well as being verbs in their base form.
Perhaps they are linked in a way I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for tarnish that I've seen before include "(Of metal) lose lustre" , "Discolour - sully" , "Spoil" , "Stain like silver" , "Stain or blemish, silver or reputation" .)

I've seen this clue in the Evening Standard.

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