I don't understand what "the rhythm of the song's melody" describes.
Sometimes when we have some complex rhythms, I have my choir just "say" (not sing) the words to the rhythm of the melody to make sure they understand the rhythm. It's not on pitch, but each word will land where it's supposed to. Or sometimes we'll clap the rhythm of the melody to get a sense of where all the notes hit.
I think that's what's being asked here. Convert that melody rhythm so that, for example, a percussion instrument just hit the rhythm of the melody without the actual notes.
That way, you might have:
Some-where o-ver the rain-bow... which is:
half-half-quarter-eighth-eighth-quarter-quarter
so the percussion instrument just hits that:
half-half-quarter-eighth-eighth-quarter-quarter
(without actually doing the melody notes).