Left Turning Tendencies
In a single-engine airplane with a clockwise-rotating propeller, several left-turning tendencies exist which can roll or yaw and airplane to the left.
Torque
Because the propeller has mass and is spinning, there is an equal and opposite reaction which is applied to the airplane.
With a propeller that spins clockwise (when viewed from the cockpit), there is a torquing moment that is rolling the airplane to the left.
P-Factor
At high angles of attack, the relative wind is hitting the propeller from above. Note that at any point when one side of the propeller is descending, the other side is ascending upside down.
This causes the downward-moving blade to be at a higher angle of attack (relative to the oncoming wind), than the upward moving blade. Just like the main wing, the higher angle of attack produces a greater aerodynamic force. In this case, this force yaws the aircraft to the left.
For this reason, this occurs mostly in high angle-of-attack flight attitudes.
Gyroscopic Procession
A spinner propeller is subject to gyroscopic procession, just like any spinning object. When a force is applied to the spinning disc, the effect of that force is felt 90 degrees from the point of application.
Thus raising the nose of the airplane is applying a force to the top of the propeller, which causes the force to be applied 90 degrees later, causing a left-turning yawing moment.
Spiralling Slipstream
Air accelerated behind the prop (the slipstream) follows a corkscrew pattern. This clockwise pattern wraps around the airplane, and strikes the tail of airplane on the left side. This causes a yawing moment to the left.