A simile center is a commonly used crossword clue; the answer is "asa" or "asan." This relates to the figure of speech where two unlike things are compared.
The crossword clue "simile center" plays on the fact that when writing a simile, the phrases "as a" and "as an" link two unlike things. "Wise as an owl" is a common simile. "Happy as a clam" is another example. In many similes, the phrases "as a" and "as an" form the center of the figure of speech. The clue hints at this arrangement to identify the short answer "asa" or "asan" in a crossword puzzle.