This link shows you how to do something similar to display EXIF and IPTC data. So I’m sure you could easily adapt that idea to show / hide the description instead.
http://wppa.opajaap.nl/iptc-and-exif-data/wppaspec/lcen_US/ab60/pt2176/cv0/oc1
The description is in tag 2#005. I’m not sure how you display only part of it. Here is the HTML I use for the description. You would just need to change the first line so it only shows so many characters of the description.
2#005<br />
<a href="javascript://" onClick="jQuery('.wppa-dtl').css('display', 'block'); jQuery('.wppa-more').css('display', 'none');">
<div class="wppa-more">
More info and Exif data
</div>
</a>
<a href="javascript://" onClick="jQuery('.wppa-dtl').css('display', 'none'); jQuery('.wppa-more').css('display', 'block');">
<div class="wppa-dtl" style="display:none;" >
Hide info
</div>
</a>
<div class="wppa-dtl" style="display:none;">
<br />
<style>
.wppa-label { padding: 0 3px !important; border: none !important; }
.wppa-value { padding: 0 3px 0 12px !important; border:none !important; }
</style>
<table style="margin:0; border:none;" >
<tr><td class="wppa-label" >Caption</td><td class="wppa-value" >2#120</td></tr>
<tr><td class="wppa-label" >Date Time Captured</td><td class="wppa-value" >E#9003</td></tr>
<tr><td class="wppa-label" >Photographer</td><td class="wppa-value" >E#013B</td></tr>
<tr><td class="wppa-label" >Copyright</td><td class="wppa-value" >E#8298</td></tr>
<tr><td class="wppa-label" >Location</td><td class="wppa-value" >2#092, </td></tr>
<tr><td class="wppa-label" >Camera</td><td class="wppa-value" >E#0110</td></tr>
<tr><td class="wppa-label" >Lens</td><td class="wppa-value" >E#A434</td></tr>
<tr><td class="wppa-label" >Focal length</td><td class="wppa-value" >E#920A</td></tr>
<tr><td class="wppa-label" >Shutter Speed</td><td class="wppa-value" >E#829A</td></tr>
<tr><td class="wppa-label" >Aperture</td><td class="wppa-value" >E#829D</td></tr>
<tr><td class="wppa-label" >ISO Speed Rating</td><td class="wppa-value" >E#8827</td></tr>
<tr><td class="wppa-label" >Exposure bias</td><td class="wppa-value" >E#9204</td></tr>
<tr><td class="wppa-label" >Exposure program</td><td class="wppa-value" >E#8822</td></tr>
<tr><td class="wppa-label" >Metering mode</td><td class="wppa-value" >E#9207</td></tr>
<tr><td class="wppa-label" >Flash</td><td class="wppa-value" >E#9209</td></tr>
<tr><td class="wppa-label" >Software</td><td class="wppa-value" >E#0131</td></tr>
</table>
</div>
