In this passage, the 1841 British edition accidentally omitted the indefinite article a before high priest. The subsequent 1849 LDS edition restored the a to the LDS text. Elsewhere the Book of Mormon has examples of high priest both with and without the indefinite article a:
For each case of “(a) high priest”, the critical text will follow the earliest text.
Summary: Maintain in Alma 13:14 the indefinite article a before high priest, the earliest reading.