Well, a good stiff belt (of leather, not whisky), is the foundation for proper carry. It will keep your pistol in proper position, help support its weight, and allow you to accomplish the draw much better without the holster binding on the pistol. I always recommend belt loops over clips to ensure that you don't wind up with a fistful of holster on the draw.
Everybody has their own opinion, and is entitled to it, but I would definitely not wear an IWB with a clip without a belt.
That being said, Cops Plus carries the Bianchi 6, made from suede for comfort, with a belt clip, for the XD-40: CopsPlus
I have not bought from CopsPlus, but just did a search. This is not an endorsement.
There are a lot of other clip-on holsters out there, beginning with Uncle Mike's and Don Hume, and go up from there.