I have a toilet water supply valve with the tank supply line permanently attached to the valve. They don't make them any more and I understand why. The valve assembly is connected to a half inch supply line coming out of the wall, behind and beneath the toilet. The nut which seems to attach to the supply line is the part in question.. . I've been told it is a reverse thread and to loosen it, it should be turned clockwise. Others have suggested it is a standard thread and will loosen in a counter-clockwise direction.

I can't afford to damage the supply line. Does anyone know for sure??

Assuming a standard compression fitting the nut would be a standard right hand thread and would be removed counter clock-wise. Remember however that you are looking at the opposite end when looking at the handle. Looking at it from the handle the nut would turn clock wise to remove. Since it seems to be critical that this not be damaged you might want to post a picture so we can be sure you don't have something unusal here.