Circle with arrow symbol interpretation in circuit
What is the meaning of the circle with arrow in it symbol, that has
5mA?

It's a constant current source - it draws a constant 5 mA come what may.

Should I interpret that as an extra power source?

You should interpret that as a current source - it can be converted to a voltage source in series with a resistor if that might help you - look up Thevenin equivalent sources: -