A problem affected one (5)

I believe the answer is:

poser

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'a problem' is the definition.
(poser is a kind of problem)

I cannot understand how the rest of the clue works.

(Other definitions for poser that I've seen before include "Problem set by the model" , "Artist's model" , "Person still being portrayed" , "Question; one trying to impress" , "Question - a sitter" .)

I've seen this clue in The Times.

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