Is "一“ in 一线城市 first or second tone?
一 in 一线 has two kinds of tones, and two corresponding meanings:


yi1 xian4 (1st tone), means front line / 1st line, such as 一线城市 (first-tier city), 亲临一线, 一线队.
yi2 xian4 (2nd tone), means a gleam of / a ray of, such as 一线光明, 一线生机.


Basically, before the word with 4th tone, 一 should be pronounced as the 2nd tone (“一”的音变), such as 一样, 一辈子. But if it is used as ordinal, should use its original tone (the 1st tone), such as 第一名, 第一线. Note in this case 第 could be omitted, but still should keep its original tone (the 1st tone). So if 一 is used as the meaning of 第一 (the 1st), it should always be pronounced as the 1st tone.

Reference:
试析“一”声调的读法
“一”的音变
“一”的变调