Whenever a crossword clue gives a quotation (and source) and omits a word, try Googling the source and the first few words of the quotation. It's there as the first entry if you Google "Alexander Pope Expression is the"
Added to that if you type in D?E?S in the Word Finder the only word that is not a plural that comes up is DRESS, so it must be the only word that would fit that blank in the quotation!
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