。
A single tear ran down her cheek.
Literal
Tear [subject-が] one-streak cheek [object-を] flowed-down.
一筋 ('a single line, a streak') is the counter 筋 ('streak, line, strip') with 一 — used for things that form a line: light, smoke, tears. Here it counts the single rivulet of tears. A more image-focused variant than the generic 'tears ran.'