Some dragons have the Change Shape feature. For example, the Ancient Gold Dragon’s Change Shape feature reads as follows:
Change Shape. The dragon magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (the dragon’s choice).
In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.
This seems to make it eligible for the Clone spell, which reads as follows (emphasis mine):
This spell grows an inert duplicate of a living, Medium creature as a safeguard against death. This clone forms inside a sealed vessel and grows to full size and maturity after 120 days; you can also choose to have the clone be a younger version of the same creature. It remains inert and endures indefinitely, as long as its vessel remains undisturbed.
At any time after the clone matures, if the original creature dies, its soul transfers to the clone, provided that the soul is free and willing to return. The clone is physically identical to the original and has the same personality, memories, and abilities, but none of the original’s equipment. The original creature’s physical remains, if they still exist, become inert and can’t thereafter be restored to life, since the creature’s soul is elsewhere.
Now, presumably if you used Clone on a creature under the effects of True Polymorph, the spell would only clone the active layer of True Polymorph, meaning that the base creature underneath would not get cloned. However, the dragon’s Change Shape feature specifically grants it the ability to transform back into its true form with an action. I suppose my confusion is just what that means for the cloned creature. Could it still turn back into its dragon-form with an action, or would its “true form” now just be its cloned form, making this action essentially useless?