。
She has a heart unblemished by impurity.
Literal
She [topic-は] dirt [genitive-の] not-existing heart [genitive-の] possessor is.
The pattern Xの持ち主 ('the possessor of X') is a common way to attribute a quality to a person — literally 'an owner of X,' but functionally equivalent to English 'a person with X.' It feels slightly more elevated than the plain ~がある, and is favored for compliments and character descriptions: 鋭い感性の持ち主 ('a person of sharp sensibility'), 強い意志の持ち主 ('a person of strong will'). 汚れの無い ('without blemish/stain') is poetic register, contrasting moral purity (here 心) with literal stains. The polite copula です gives the sentence a faintly formal, almost ceremonial weight that suits high praise.