Certainly, you can say that F=ma is a definition rather than a law, i.e. the force on an object is defined to be the acceleration it would produce (in the absence of any other forces) times the mass of the object.
A definition, in itself, cannot say anything substantive about the universe. However, there's something else: Force, defined this way, is a useful concept. For example, if two forces are applied independently to the same object then you can get the net force by vector addition; the force of a compressed spring pushing on an object is independent of the color or size or shape or mass of the object it is pushing on; etc. etc. If you defined force in some stupid way, like
force = (mass)×(RGB color of the object)×(Greenwich mean time)
or whatever, then force would be a dead-end not leading towards anything useful, and it would not have any of these properties that we expect forces to have.
In other words, all the "obvious" properties of force, too obvious to say explicitly, like the fact that the force of a spring can be fruitfully discussed without knowing what the spring is pushing on, are really part of the broad interpretation of Newton's law F=ma. And they are truly substantive facts about the physical universe.