。
Her short temper is her one flaw.
Literal
She [topic-は] mind [subject-が] short [nominalizing-のが] flaw-on-jewel is.
玉に瑕 (also written 玉にきず) is a compact proverb — 'a flaw in the jewel,' meaning a single defect that mars an otherwise excellent thing or person. The image is a perfect gem with one tiny crack. Pair it with 気が短い ('short-tempered') and the construction ~のが [predicate], and you get a polished single-clause judgment: 'her short temper is the one flaw.' 玉 ('jewel, sphere') is itself a productive metaphor in Japanese — 目玉 ('eyeball'), 玉子 ('egg'), 玉手箱 ('treasure box'). The の nominalizes the clause 気が短い for use as a subject.