"Piece of cake" or "pie in the sky" (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
idiom
(Other definitions for idiom that I've seen before include "Form of linguistic expression" , "Colloquial saying" , "'Break a leg' or 'miss the boat'" , "Dialect, vernacular" , "Vocabulary characteristic of a specific group" .)