Can I override a #id ul li behaviour with a class definition

I have an area that is identified by a #id and there is a CSS like:

#id ul li {
    margin:0;
}

can I, for a specific UL in that area, override the margin-setting? I understand that #id creates very high priority in evaluating the formatting.

I have tried:

.myclass ul li {
    margin-left: 20px;
}

and

#id ul.myclass {

as well as

#id li.myclass {

Is it even possible?