一筋
ひとすじ one line; one stretch (e.g. of road)
Why it means this
一 (one) + 筋 (a line, a thread) — a single streak or line (一筋の涙); as a suffix, 'devoted solely to' (研究一筋; 仕事一筋).
Breakdown
Each character links to its full entry — origin story, stroke order, and readings.