i noticed that the clutch didn't work one you are in low speed. in low speed for exemple 10km/h if you use the clutch the speed remain in 10km/h like city car driving. but in this game this option is note included even ets2 or ats2 or simulator bus....etc this option is not included.
i dont think you can use the clutch properly in arcade mode, Hit 'Q' and switch to realistic mode, and press the 'Shift' key to use the clutch, better to assign an analog control if you're on a wheel or controller.