。
She's a sharp-minded person.
Literal
She [topic-は] head [possessive-の] sharp person is.
さえた is the past/resultative form of 冴える ('to be sharp, clear-headed'), functioning here as an adjectival modifier: 頭のさえた人 = 'a person whose mind is sharp.' This prenominal pattern — [noun]の[verb-past]人 — is a natural way to describe people by a resulting quality, similar to 目の大きい人 ('a person with big eyes').