There isn't a way currently to apply a priority.
If you know that your description is in a particular field I would suggest adjusting the metadata mappings and either remove the fields that you don't use, or map the field that you are interested in to a separate metadata field (e.g. searchDescription) then use this in your search results template.
If you must assign a priority you'll need to map each metadata field to a separate class and then use conditional logic in your template to handle the order of display.
e.g.
<#if s.result.metaData["twitterDescription"]??>
${s.result.metaData["twitterDescription"]}
<#elseif s.result.metaData["ogDescription"]??>
${s.result.metaData["ogDescription"]}
<#else>
${s.result.summary}
</#if>