Learn Dutch: De docent gaf een heel duidelijke definitie van het woord. - The teacher gave a very clear definition of the word.

You can tell by its suffix! Nouns ending in -ie are almost always de words. These are often words borrowed from Latin or French, like de politie, de informatie, and de definitie.

Is there a similar suffix rule that tells us docent uses de?

Yes! Nouns ending in -ent that refer to people are always de words. You likely already know examples like de student and de patiënt; de docent follows the exact same pattern.

Why does duidelijke get an -e ending here, even though it comes after een?

Because definitie is a de word. Adjectives placed directly in front of a noun get an -e added to them. The only time you leave the -e off is when you have a het word preceded by een (or no article). Since we know it is de definitie, the adjective must be duidelijke.

Why doesn't heel get an -e ending like duidelijke does?