Dynamically adding actionscript to a movie clip

hey,

Well… I have this movieclip… and i’ve attached it to the stage with this…

_root.attachMovie("Item", "Item", 0, 0);

How do i go about adding some actionscript to this?

the actionscript i wanna add is pretty simple… it’s this…

onClipEvent(enterFrame){
this._y++;
}

Soo… How should i do this?

Thanks