In CF5, when deleting from a Verity collection, you could pass a value as the key, like so:

However, in MX, you MUST pass key as a column name and pass a query in. This means if you want to delete one item, you need to create a temporary query to store the value. The syntax would look like so:

(note, "key" above is a variable that represents the key you want to delete from the index)