Hello, if you use the version 2 of Librecad, here you have the solution, for previous versions it should be something similar:

let's suppose you have the picturedown here

first you need to select modify>divide

below the drawing area, the command line asks the entity to divide (I cutted and pasted to show)

click on the circle, now notice that the circle has changed color and the command line asks a poit to divide

click point A using snap bar

now click again modify>divide, click on the circle and click on point B

and of course the final result (I clicked on the cutted part to show to you)

That's all friend.

Bye!

Claudio

...the problem is not the problem, the problem is your aptitude about the problem...