一線之隔

a fine line / a fine distinction

one / single / a (article) / as soon as / entire; whole; all; throughout / "one" radical in Chinese characters (Kangxi radical 1) / also pr. [yao1] for greater clarity when spelling out numbers digit by digit

front line

thread / string / wire / line / CL:條|条[tiao2],股[gu3],根[gen1] / (after a number) tier (unofficial ranking of a Chinese city)

(possessive particle, literary equivalent of 的[de5]) / him / her / it

to separate / to partition / to stand or lie between / at a distance from / after or at an interval of