A clutch of hydraulic-pressure type comprising a linearly controlled piston (300) that is movable in a channel in the clutch and that carries a pivotally mounted support leg (208) that is spring biased into axial alignment with the piston, the side of which facing away from the support leg is loaded by the hydraulic pressure. The clutch is annular and co-operates with a shaft shank (1), and the free end of the support leg rests against a support surface carried by the shaft shank (1) upon rotation between the shaft shank (1) and the clutch (4), the support leg is turned away, whereby the piston (300) is displaced and opens a valve through which the hydraulic pressure is evacuated.