Puzzle #51

Anne Franck, famously - D(I)ARIST

Somewhat desperate - DRASTI(C)

Motorcyclist's "back seat" - SID(E)CAR

Mystery novel poison - ARSE(N)IC

Positive - CER(T)AIN

Not as precise as it could be - INE(X)ACT

Worse off that "endangered" - EXTINC(T)