Gaining muscle mass requires a caloric surplus.
Losing fat requires a caloric deficit.
You can clearly see the contradiction. It isn't going to work, and especially not if you train for mass only once a week. You see, if you're in a caloric deficit, meaning you consume less than you expend, your body will have to provide the missing calories itself to maintain function. That energy is going to come from fat stores and possibly muscle, should protein consumption not prove sufficient for things it considers more vital (such as, say, keeping your liver running). Building muscle mass is an anabolic process which will require energy, thus calorie intake. If you are in a deficit, it is not a priority for your body.
Now, from this it is maybe not unreasonable to believe that it could be possible to build muscle and lose fat at the same time. After all, if you provide a stimulus to your body in the form of heavy weightlifting that signals it that its current state is inadequate so it must improve muscle tissue, while at the same time being in a slight deficit, maybe there's no reason to believe our biology isn't clever enough to invest in muscle tissue and burn excess fat (which is there as an energy store anyway). You'll find sources out there that state this is indeed possible for beginners in strength training, and that with a careful calorie and nutrient balancing you can "recomp": slowly lose fat while also slowly building muscle.
The problem with all of that is, that it's mostly hearsay. Conventional wisdom can be wrong, and just because something is repeated enough doesn't make it true. Even if it were true, progress in this way would be slow. What you can be sure of, however, is that a caloric deficit will result in weight loss, and that a caloric surplus (at least on top of your total daily energy expenditure without considering training and additional protein synthesis) is a requirement for muscle gain. So rather than gamble, you might want to go with what has been proven time after time to work. Focus on one goal, then the other.