Being unrealistic about claims made by trader and performer? (10)

I believe the answer is:

idealising

I'm a little stuck... Click here to teach me more about this clue!

'being unrealistic' is the definition.
(being idealistic)

I cannot understand how the remainder of the clue works.

(Another definition for idealising that I've seen is " wearing rose-tinted spectacles?".)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.