Forgot that I hadn't finished 20D either, because it didn't make sense at the time.
Your questions triggered another look, then suddenly an epiphany.
I know I'll fail = DEFEATIST
with no mark short of failing -> letters E & F, remove D & A (?)
vocally [homonym]
most worn out = EFFETEST (not in my book)
effete = exhausted (Chambers def 1)
Yeah, a rather lame clue.