I've used audio mixer to adjust volume level of a clip on 2nd. audio track.
After this the level control in the audio mixer is staying at its lowest position even after I return it to 0 and this is affecting other clips on this track on the timeline.
Can you tell me how to use audio mixer to adjust one clip on the timeline without affecting others
You don't. The Mixer affects the entire track. If you want to adjust a single clip, you have to work on it directly, outside the Mixer.