How do I return only a truncated portion of a field in SOLR?

What you're looking for is the highlighting hl.maxAlternateFieldLength (http://wiki.apache.org/solr/HighlightingParameters#hl.maxAlternateFieldLength).

You will need to define the field as its own alternate field. If you want to highlight the field Description, the highlight query parameters would be:

hl=true
hl.fl=Description
f.Description.hl.alternateField=Description
hl.maxAlternateFieldLength=300

Finally, to omit the Description field from the query result, you will have to exclude it from the fl query parameter:

fl=score,url,title,date,othermetadata