Is there any way - an effect I can use - to clamp negative colors to zero/black?

It's easy to end up with colours dipping below zero/black - just lowering the Shadows exposure of a clip often does it, as you can see on the Waveform video scope. Negative values for colours aren't necessarily a problem if there's nothing being composited over the clip (they'll end up being clamped to zero on export), but if there is something above, especially something Additive like a glowing logo, those negative values on the clip beneath end up altering the colours of it.

I need some way to either tell the Color correction tool(s) to clip Blacks at zero, or an effect I can drop on that'll clamp them instead. I often have to do this when comping in other packages (the Levels effect in AE has options to clamp blacks and/or whites). 

FCP is so fast for doing final tweaks to things, and for compositing layers of graphics together, but I can't tweak colours much without this problem coming up. Is there an effect or a checkbox I've missed?

"Broadcast Safe" effect?

Well, I'm not quite sure I understand the logic of the Broadcast Safe parameters, but yep — that seems to work! Thanks!What I don't get about that effect is that it has an "Amount" parameter, range from 0 to 10. You'd think that if you add an effect to a clip and set its Amount to zero, that it'd have ... zero effect.But that's not the case here. Set it to zero, and it has the effect of clamping sub-blacks and over-whites, and leaving all other colours where they are. (Which is just what I needed, yay!)If you do dial the Amount up, it clamps and then rescales the incoming luminance from full to video levels (so from 0-255 to 16-235 for 8-bit Rec.709). Dare say it may correct illegal colours too, but I wasn't checking. But that all makes sense.Anyway this is a very long way of saying thanks tl;dr: To clamp blacks and whites, drop on a Broadcast Safe effect, leave the Amount slider set to 0, and buy @DavisS a coffee.