adjustment layer to affect only the layer immediately below it

E.g. a 4K clip in a HD timeline turns the 4K into an HD nest: no room to move about.

I usually put the clip in a 4K timeline do what ever has to be done: nest and put the 4K nest in the HD timeline.

 the nested sequence alters the nature of the clip

By @marcosg77763356

I did upvote your request, but how does it do that?

I can understand why you would want this but Ps works quite different from Pr.

Don't think this is feasible.

Yes, this is the workaround we have used so far, but it is not exactly the feature one is looking for, and the nested sequence alters the nature of the clip. It would be much more flexible to have this simple feature readily available. Furthermore, to then access the effect settings, you lose the context in a nested sequence. In other words, the nesting solution only works for simple cases but does not provide the flexibility to select the layer to be affected. 

Nest the AL and the clip.