If you first wipe the piano wire end with alcohol ( don't use rubbing alcohol ) to remove oil, debris etc. Then use fine or medium sandpaper on it which creates a better surface for the solder to grab onto, then wipe it clean again with the alcohol before you solder. Try not to touch the wire end again with your fingers, which might transfer oil from your skin onto the area which is to be soldered.

Bring both wire and clevis to a high enough temperature which allows the solder to flow, avoiding what is called a "cold joint".

Clean the completed assembly with a soft wire brush, and work a drop of oil all around it.

Take a photo and post it, if you wish. I appreciate good solder joints! ( Your plane will too! )