Confirmed by the Senate on 25 July 1913 as a second lieutenant, Field Artillery (H-C02). Page as printed.
Image · dated 25 Jul 1913 · source: Internet Archive (public-domain scan); sim_congressional-record-proceedings-and-debates_june-17-1913-august-2-1913_50

How it is shown: Complete page image as saved; no crop, no retouch.
Keep in mind: Confirmed as printed (H-C0x). The page lists many officers; only Herbert Slayden Clarkson is tagged. Printed list, not proof of his service.
Evidence for
- Congressional Record, 25 Jul 1913, p. 2756: nominations confirmed by the Senate, Field Artillery Arm, to be second lieutenants · the original at U.S. Congress, Congressional Record (Internet Archive scan)