I recommend against any DM allowing it, but a (weak) case could be made
Alternate Source Spell and Southern Magician are two feats that allow you to use one type (arcane, divine) of spell as the other type.
The idea here is to get your cleric class to count as both an “arcane spellcasting class” and a “divine spellcasting class” and use mystic theurge to advance it twice per level. This is pretty much universally ruled invalid, despite the rules being admittedly unclear.
Anyway, Southern Magician is from Forgotten Realms, and requires you to be a Mulan human. Since you worship a Forgotten Realms deity, and not one usually worshiped by Mulan, I’m guessing you don’t qualify. Moreover, Southern Magician contains the following line, which Customer Service used to argue that it could not be used to enter mystic theurge:
The actual source of the spell's power doesn't change, nor does its means of preparation. You are merely weaving the strands of magic together in an unconventional way that makes the spell behave somewhat differently.
Alternate Source Spell is harder to get (you have to already have both arcane and divine spells before you can take it), but it lacks both the racial requirement and blurb about “power source” from Southern Magician. So you could, assuming your DM is insane and allows this, dip a level of some arcane spellcasting class, losing yet another cleric level, and then take three levels of mystic theurge to make up for that and the levels lost to a previous prestige class. And, potentially, keep going for another seven levels, rocketing past where a cleric is supposed to be, breaking the game even more than a regular cleric does.
For the elemental savant in particular...
One could argue that, in the case of a Cleric 5/Elemental Savant 10, taking a single level of some arcane class and then four levels of mystic theurge isn’t really broken. After all, at 19th level you wind up with exactly the spellcasting of a 19th-level cleric, and only at 20th do you jump ahead of a regular cleric to 21st (which is meaningless because even under the Epic rules the 21st level of cleric does not grant any improvements to spellcasting beyond caster level).
And this is certainly true, but it’s a wrong solution to the problem. It sets horrible precedents, requires a bizarre dip in some random arcane class, and then mystic theurge isn’t actually doing the thing that mystic theurge claims to do on the tin.
A better solution is to just houserule the elemental savant to give a 10/10 spellcasting. Elemental savant is a rather mediocre class; even the capstone, which is pretty good, has a major drawback for you (since you also pick up vulnerability to cold). Is it something for nothing? Not quite (Energy Substitution isn’t that great a feat), but effectively, yeah. But that’s also the nature of the game; spellcasters have lots of options to get something for nothing. If it really bothers the DM, leave the capstone as not progressing spellcasting; I can’t recommend taking it at that point, but certainly nothing prior to 10th grants anything notable.