Poser Common

a person who habitually pretends to be something he is not

"Her friend was nothing more than a poser, constantly acting like an environmental activist while secretly supporting deforestation projects."

a person who poses for a photographer or painter or sculptor

"the president didn't have time to be a model so the artist worked from photos"

a particularly difficult or baffling question or problem

"The budget shortfall turned out to be an impossible poser for the city council during their emergency meeting."

A particularly difficult question or puzzle.

"The crossword clue about ancient Greek philosophers turned out to be quite a poser for everyone in the group."