I have a clip in my sequence that is very bright, at say 6 seconds into this clip I want the brightness to return back to normal until it reaches the end of the clip. So what I thought I had to do was add a keyframe at the bit I wanted the brightness to start animating back and one at the end of the clip and simply choose brightness and contrast from the clip animation options, then adjust the yellow line up and down. But adjusting the yellow line moves the whole line, not just from where the keyframe begins, therefore it affects the whole clip, not the bit in between the keyframes.
Also, I am trying to frame hold it at the first keyframeexcept I don't have the option to hold it from here, just from in point, out point or marker 0. I tried adding a marker but still I have no option to hold it from my own marker.
Use more keyframes. You can only drag the yellow line between keyframes, but in your case the first keyframe does not budge because there are not enough keyframes. Insert another keyframe in the frame before the current keyframe and then you are able to drag the yellow line.
To drag the yellow line you need a beginning and an ending keyframe in the clipsegment you want to modify. You have not defined them.
You may have better luck adjusting the the keyframes's values in the effects control window.