。
She is a clerk at a supermarket.
Literal
She [topic-は] supermarket [genitive-の] clerk is-formal.
~である is the formal/written copula — replacing だ in essays, news articles, academic prose, and other elevated writing. It's not appropriate for everyday speech (where it would sound stiff or pretentious). 店員 ('shop clerk / sales staff' — literally 'shop member') is the standard word for retail-floor employees in Japan; supermarkets, convenience stores, and department stores all use the same term. スーパーマーケット ('supermarket') is the unabbreviated loanword form; the everyday spoken form is the shortened スーパー.