loaded into loaded div

sorry if this has been asked before, but everything I search just gets me how to load HTML into a div that's already there.

I have a div

<div class="compIn modalFill">
</div>

into which I load an HTML file with some content including

<div id="field">
</div>

I want to load another HTML file into this div, however I am struggling to figure out how to trigger this

thanks in advance.