I am not a teacher.
The prefix "a-" in that sense makes hyphenated words: a-knocking. The American Heritage Dictionary has a useful note on the subject.
The AHD also has a note about that. I, personally, hate the semicolon with those. If you don't like it in first position, and neither do I, you can start a new sentence and embed the qualifying adverb where it really belongs: "His horse came in third. He, however, had won the only race that mattered, the race for Belinda's heart." (What do you want for nothing---F. Scott Fitzgerald?)
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