My best guess is that it ends with "short."
(Other candidates include: support, retort, report, resort and venereal wart.)
Of course, it'd have to be six single-syllable words, considering there are six syllables in that final line and the powers that be say there are only six words left unheard.
"There once was a wizard named Voldemort,
Who partied all night in Hyannis Port.
He awoke feeling sore,
Pulled out twelve dollars more,
And said, 'Madam, if you'd help, I feel short.'"