Basic piano installed in flat (6)

I believe the answer is:

staple

(Other definitions for staple that I've seen before include "Main, important" , "security device" , "Fastener - principal" , "Paper fastener found on plates" , "standard" .)

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.