Some dragons have the Change Shape feature. For example, the shape change feature of the ancient gold dragon reads as follows:
Change shape. The dragon magically transforms into a humanoid or beast whose challenge is not greater than his own or his true form. He resumes his real form if he dies. Any equipment that he wears or carries is absorbed or worn by the new form (the choice of the dragon).
In a new form, the dragon retains its alignment, health, dice, ability to speak, skills, legendary resistance, actions, scores in Intelligence, Wisdom, and Charisma, as well as that action. Its statistics and features are replaced by those of the new form, with the exception of class features or legendary actions on this form.
This seems to make it eligible for the Clone spell, which reads as follows (that's me pointing out):
This spell develops an inert duplicate of a living creature and average like a
protect against death. This clone forms in a sealed vessel and
reaches its size and maturity after 120 days; you can also choose from
have the clone be a younger version of the same creature. He stays
inert and lasts indefinitely, as long as his ship remains
At any time after the clone, if the original creature dies,
his soul is transferred to the clone, provided that the soul is free and
ready to come back. The clone is physically identical to the original
and has the same personality, the same memories and abilities, but none of the
the equipment of origin. The physical remains of the original creature, if
they always exist, become inert and can not be restored later.
life, since the soul of the creature is elsewhere.
Now, probably if you were using Clone on a creature under the effects of True Polymorph, the spell would be only clone the active layer of True Polymorph, which means that the base creature below will not be cloned. However, the Change Shape function of the dragon gives it specifically the opportunity to reconvert itself into its true form with an action. I guess my confusion is just what it means for the cloned creature. Could he still return to his dragon form with an action, or would his "true form" now only be his cloned form, rendering this action essentially useless?