You're going to need to use arc() in combination with a little bit of trigonometry to draw the head...
Something like this?

(This is sort of a proof-of-concept sketch that I made)

1. void setup() {
2.   size(400, 400);
3.   smooth();
4. }
5. void draw() {
6.   background(255);
7.   circArrow(200, 200, 100, 0, radians(mouseX % 360), 10);
8. }
9. void circArrow(float x, float y, float radius, float start, float stop, float arrowSize) {
10.   ellipseMode(CENTER);
11.   noFill();
12.  
13.   arc(x, y, radius * 2, radius * 2, start, stop);
14.  
15.   float arrowX = x + cos(stop) * radius;
16.   float arrowY = y + sin(stop) * radius;
17.  
18.   float point1X = x + (cos(stop) * radius) + (cos(stop - radians(arrowSize * 5)) * (arrowSize));
19.   float point1Y = y + (sin(stop) * radius) + (sin(stop - radians(arrowSize * 5)) * (arrowSize));
20.  
21.   float point2X = x + (cos(stop) * radius) + (cos(stop - radians(-arrowSize * 5)) * (-arrowSize));
22.   float point2Y = y + (sin(stop) * radius) + (sin(stop - radians(-arrowSize * 5)) * (-arrowSize));
23.  
24.   line(arrowX, arrowY, point1X, point1Y);
25.   line(arrowX, arrowY, point2X, point2Y);
26. }

Thank you, just have to think how you computed the x/y coordinates ;-)