Assuming you use the Author field as the “word” you can either change the Reference Type to reflect that and use the following in the Dictionary template (where the * is replaced by the “link adjacent” symbol. Or leave Word as Author and don’t bother with the Ref Type edit. (or use the title for the word and adjust the template accordingly).
Attached is an amended style APA 6th-dictionary.ens where the Ref Type for Dictionary was changed to replace the Author field name as Word.
Just a comment about the APA format for online dictionaries. The format provided on the Merriam-Webster.com site (per alexl’s first posting) isn’t following APA 6 guidelines as retrieval dates are no longer needed and the url link should be placed last: