I don't think melody is "the" defining element of music, so much as one among a number of defining factors. For me, music's anything I choose to listen to as music.

But even in purely conventional terms, to foreground melody is to forget about percussion-led music. A drum solo is music, even if it's not pitched notes. (Though of course drum hits do have a pitch of sorts - as rendered explicit by drumers of the calibre of Max Roach.)

Most hip-hop does have some kind of tune behind it, especially the commercial chart stuff. But even the hardcore, minimal stuff that's little more than drum hits, a bit of bass and a 'spoken' vocal is still undoubtedly music. To suggest otherwise is like saying a samba troupe isn't music, or that a tabla player isn't playing music.