Thinking about philosophical issues in terms of definitions is a mistake. Definitions are useful as abbreviations for discussing explanations when you understand the relevant explanation. So then the issue is what role does free will play in our explanations?
If a rock falls off a mountain and kill somebody nobody will prosecute the rock. If a person pushes a rock so that it falls and hits somebody then we may prosecute that person because he had free will. What is the relevant difference? The rock will fall if and only if it is set up in the right way. Nothing we do to the rock can change that. By contrast, how a person will act depends on what knowledge he has and it is possible for him to learn different ideas than the ones he has now - to change his mind. If he has not learned that murder is a bad idea then there is some flaw in the ideas that he enacts that stopped him from learning. And until he learns differently we may reasonably expect that he might murder somebody else. Determinism being true doesn't stop people from having ideas and acting on them. It doesn't stop us from talking to people who do stuff that is bad or foolish, or from locking them up if they pose a threat.
At this point the usual objection is "but that's not what people mean when they say free will". The problem with this objection is that you still need a word to distinguish between physical objects that can learn and those that can't. You can invent a new term if you want but I don't see the need to refrain from using the term "free will" as an abbreviation for the explanation I gave above.