Yeah, there's a million ways you could do this, really. Which sim(s) are you talking about? If it's GTR2, GT Legends or Race 07, try mapping a keyboard key to the clutch and see what the in-menu input bar graphs do, I'm sure that when you press the clutch key it'll engage fully instantly but slowly drop off when you release the key. I don't know how to set it to do that in sims that don't do it automatically, but that's good enough to prevent stalling, if less realistic.