Consider by analogy, if I ask "May I have the apple?" You might think that I should have asked, "May I have an apple?" since there are many apples in the world and it shouldn't matter which one I receive. But if you are holding one apple, and it is the only apple within sight, then using the definite article "the" is entirely appropriate. I am asking for that specific apple, and none other. I could add "May I have the apple that you are holding?" to be perfectly clear, but I don't necessarily need to.
It is the same with the situation you describe, where you ask "What is the meaning of word X?" where X certainly has multiple possible meanings across all possible uses in speech and writing. However, it will be understood that you are referring to the one specific meaning currently in scope. If you did want to be more clear, you could add, "What is the meaning of word X as you have just used it?" but it is not necessary.