You need to compute the point on the circle and then the point that lies outside of the circle. Then use beginPath, moveTo, lineTo and finally stroke to draw the line.

To compute a point at distance d from a point of coords (x, y) and angle an you can use the following formula:

(x + Math.cos(an) * d, y + Math.sin(an) * d)

Now you compute 2 points, one using d as the radius of the circle, and then d as something bigger than radius, depending on your needs.

I'm assuming that the an is in radians, if it's measured in degrees then you can convert it like this:

const rad = an / 180 * Math.PI

I've cobbled this quickly together:

var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");

//draw a circle
ctx.fillStyle = '#4A8';
ctx.beginPath();
ctx.arc(250, 250, 50, 0, 6.282);
ctx.fill();

//draw 12 lines
for(let i = 1; i <= 12; i++) {
    let a = 360 / 12 * i
  drawLine(a)
}


function drawLine(angle) {
  /*
  parametric form from wikipedia:
  https://en.wikipedia.org/wiki/Circle#Equations

  x,y are origin, r is radius a is angle
  x = cx + r * cos(a)
  y = cy + r * sin(a)
  */
  
  ctx.beginPath();
  ctx.lineWidth = 2;
  //thanks Radu!
  const rad = angle / 180 * Math.PI

  //250 is the center of the circly both for x & y
  //starting point on the circumfence of the circle
  let x0 = 250 + 50 * Math.cos(rad)
  let y0 = 250 + 50 * Math.sin(rad)
  //endpoint of the stroke further outside with a length of 100
  let x1 = 250 + 100 * Math.cos(rad)
  let y1 = 250 + 100 * Math.sin(rad)
  
  //draw from x0 to x1
  ctx.moveTo(x0, y0);
  ctx.lineTo(x1,y1);
  ctx.strokeStyle = '#ff0000';
  ctx.stroke(); 
}

https://jsfiddle.net/rokt0v51/

Result: