[RPG] Does a Clone spell work on a (True) Polymorphed creature? If so, to what effect

The wording quoted in the question is from an earlier print of the PHB. In later printings, and in the SRD (v5.1, p.188) it reads:

The transformation lasts for the duration, or until the target drops to 0 hit points or dies. If you concentrate on this spell for the full duration, the transformation lasts until it is dispelled

This wording seems to answer the question, that "lasts until it is dispelled" replaces the ending conditions given earlier, but also notes that the effect can be dispelled.

For some reason, this change was never listed in official errata, though the wording did indeed change between printings.

Also, apparently Crawford stated explicitly that this clarification was made on Twitter.

Unless the caster's concentration is broken, the curse remains in effect¹

While the creature is in its polymorphed form, the saving throw is based on the new form's mental stats (Wisdom).

(PHB p. 266) The targets game statistics, including mental ability scores, are replaced by the statistics of the chosen beast.

Once it returns to its original form, as long as the caster's concentration has not been broken another save does not get rolled (except for option 3, see PHB p.218). The curse (general) remains in effect as long as the caster's concentration lasts, since polymorph's spell text does not indicate a "re-roll" of saves once reverting to original form.

In case 3, (the "do nothing each time you miss your Wisdom save" option) once the target returns to its normal form its saves should be with its normal Wisdom modifier. The text does not indicate that the curse ends if that save succeeds, only that the cursed creature does not get stuck "doing nothing" for that turn.

Debuffing

This does indeed benefit the curse's caster in the case where the target's normal wisdom is higher than in beast form. That said, the creature gets a save versus polymorph (Wisdom) in the first place, if your party is trying to change its shape. That means that the creature's first line of defense, its Wisdom save, is against the polymorph spell. If the creature had polymorphed itself, then that's not a problem.

¹ Of course, if the spell is cast with a 5th level or higher slot, the concentration requirement is no longer applicable (PHB p. 218)