Fusion research is still in its infancy. I really don't see it happening in my lifetime or what's left of it. Inertial confinement or magnetic confinement right now seems to be the most promising, but will take years of development before it shows even a glimmer of scientific efficiency and years after that to achieve engineering efficiency. Don't much care for riding the shock wave of the H-bomb. I'll take nuclear-electric over that any day.
So, working with known physics and mechanics, let's assume we have a small enough fusion reactor, with confinement of a 15 million K plasma. You are getting a lot of power out a hydrogen feed into it. This in itself will not move a spaceship. Does the super hot product helium provide thrust? If so, how would that be conducted. These temperatures are way too high for anything, with the exception of confinement fields. Somehow, you must deal with the ultra-hot reactor product stream, and thrust seems a must, so to say.
Could "direct reactor product propulsion" be designed? It seems highly unlikely you could sustain such a mechanism. At least not in this century. It all seems to boil down to confinement technologies and power requirements. The more it is considered, the more it reminds me of Rube Goldberg's fabulous inventions........but perhaps not as fanciful. It certainly doesn't seem impossible with enough effort.
Short version : The power of fusion must somehow be converted into a mechanism of propulsion. This requires mass expulsion in the opposite direct you wish to move. And this might be a high temperature "ejecta" plasma for propulsion, but from what "fuel"? If not the product stream, something else?
Or do you resort to something more mechanical, like an "atomic electromagnetic rail-gun". perhaps throwing ionized atoms of some element down specialized "rails" at relativistic speeds. Not easy to see how you obtain a propulsion mechanism from fusion without throwing out mass in a major way to really get moving. Sacrificing parts of the ship as you go has unique benefits, especially for one-way trips!
en.wikipedia.org