So front derailleurs are either clamped on or they attach to a braze-on. I’ve never owned a bike with a braze-on, so mine are all clamped on.
What type of front derailleur do you need?
If you have a braze-on frame you need a braze-on derailleur. If you don’t have a braze-on, you can either shop for the specific front derailleur you want with the correct size clamp or buy a braze-on derailleur and use an adapter clamp. The clamp has to fit the diameter of the seat tube. My Surly Cross check for example uses a 28.6mm clamp. Here is the one I’m using:
This one is nice because you can have it clamped to the seat tube and still adjust because it has the long slot that allows it to be adjusted vertically. Here is a nice one you can get on Amazon:
Origin8 DER Part OR8 BZ-ON M/F CLMP ADPTR 28.6SL – AMAZON
Be sure to get the right clamp size.