I've set up a clip to be draggable on press and dropped on release - no
problem.

on (press) {
this.startDrag();
}
on (release) {
stopDrag();
}

How do I confine the drag to just the x axis? I'd like the object to slide
horizontally and keep the same vertical depth.

Thanks in advance for any help.

myMovieClip.startDrag(lockCentre,L,T,R,B)
set lockCentre to true if you want the clip to be locked to the mouse
position.
L = left limit (in pixels)
T = top limit
R = .... and so on

so you need to set your top and bottom limits to 0:

on (press) {
this.startDrag(true,-100,0,100,0);
}

this allows you to drag the movieClip +/- 100 pixels from where you started

hth,
Ed Massey