。
She has a habit of biting her nails.
Literal
She [topic-は] nails [object-を] bite habit [subject-が] exists.
~癖がある ('to have a habit of') is the standard way to express habitual behavior in Japanese. 爪を噛む ('to bite one's nails') modifies 癖 as a relative clause. The construction is [verb plain form] + 癖がある. 癖 can carry a mildly negative nuance — it often refers to habits one should ideally break.