Spoiler alert.  The original poem is followed by the solution.


"Why Mr Holmes! Come quick! The vicar's dead!"
"Dearest Lestrade! Another killer lost?"
"The Reverend Green alas was killed in bed,
The frightened Mrs White mirrors a ghost!

Mrs Peacock is in quite a shock,
The Colonel Mustard is attending her;
Motive remains unclear, although the clock
Was stopped at six, when Mr Black was here

He burned the mail, perhaps it held a clue,
The man then ran, and no weapon was found;
Miss Scarlet who was sleeping, slept right through;
Such a tough case, so care to stake a pound?"

"Lestrade! To take your cash would be a crime!
One wonders why the clock stopped at that time!"

Who murdered poor Reverend Green, why and how?

CLUE: the solution contains 15 words.

CLUE:
    “I say old chap, those kids in Baker Street
    They’re running and a skipping: SHOO AWAY!”
    “Dear Dr. Watson, rest your weary feet!
    Perhaps you’ll learn something from childish play!”



SOLUTION

"Why Mr Holmes! Come quick! THE vicar's dead!"
"Dearest Lestrade! Another KILLER lost?"
"The Reverend Green alas WAS killed in bed,
The frightened MRS White mirrors a ghost!

Mrs PEACOCK is in quite a shock,
THE Colonel Mustard is attending her;
MOTIVE remains unclear, although the clock
WAS stopped at six, when Mr BLACK was here

He burned the MAIL, perhaps it held a clue,
THE man then ran, and no WEAPON was found;
Miss Scarlet who WAS sleeping, slept right through;
Such A tough case, so care to STAKE a pound?"

"Lestrade! To take your cash would be a crime!
One wonders why the clock stopped at that time!"


The solution is a simple skip sequence (hinted in clue 2), every sixth word is taken to obtain the solution.

*THE-KILLER-WAS-MRS-PEACOCK
THE-MOTIVE-WAS-BLACK-MAIL
­THE-WEAPON-WAS-A-STAKE