。
She's regarded as the leading authority on this issue.
Literal
She [topic-は] this issue [regarding-に関して] foremost-person [as-と] is-regarded.
A formally weighted, news-style sentence. 第一人者 ('the foremost person, the leading authority') is a Sino-Japanese compound — 第一 ('first, foremost') + 人者 ('person, expert') — used for someone widely considered the top in a field. ~に関して ('regarding, concerning') is the more formal counterpart of ~について; it shows up in news, legal, and academic writing. 目される ('to be regarded as') is the passive of 目する ('regard as'), itself a literary verb most often encountered in this passive form. The と marks the role being assigned. The whole sentence has the feel of a profile or news article framing someone's reputation.