Roosevelt's 1945 replacement (6)
I believe the answer is:
truman
(Other definitions for truman that I've seen before include "Harry S ... was 33rd US President" , "Harry S . . . . . ., American president who ordered the dropping of the atom bomb" , "US president, 1945-53" , "- Capote, US novelist" , "American president who ordered use of atomic bomb" .)