One easily painted? (6)

I believe the answer is:

sitter

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'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I don't understand the remainder of the clue.

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(Other definitions for sitter that I've seen before include "Person posing for a portrait" , "broody type" , "easy target" , "one can't stand?" , "A poser" .)

I've seen this clue in the Evening Standard.

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