Can a creature cast the spell shocking grasp and hold that spell's charge then, next round, use the reserve feat Clap of Thunder (Complete Mage 40) to make a touch attack that deals the shocking grasp spell's damage plus the Clap of Thunder feat's damage?
So this basically comes down to how pedantic you want to be—this is pretty much one of those “well, the rules never say you can’t” situations.
The issue is that the rules for holding a charge are written in a way that is very specific to spells—not to any other kind of magical touch attack. They only allow you to hold spell’s charges, and are only interfered with by casting another spell. Spell-like and supernatural abilities go unmentioned; spell-like abilities might work out due to functioning like the named spell, maybe, but supernatural abilities generally not.
In my experience, this is usually chalked up to poor editing: the rules were written in Player’s Handbook, for players, whose core classes didn’t really offer significant spell-like or supernatural abilities (the paladin’s lay on hands is literally the only supernatural touch attack offered in the book). The book is trying to explain magic to the magic-using classes—spellcasters. Most that I have played with have ruled that spell-like and supernatural touch attacks can also hold charges—and can also interfere with other held charges.
However, strictly speaking, those rules are for spells, specifically. If that’s going to be the case, then I think a DM should be consistent about it—if you can’t hold supernatural touch charges, then you can use supernatural abilities without interfering with a held charge.
And if you can do that, then the touch with the supernatural ability definitely also discharges the held charge, and then your trick works. So ask your DM about how non-spell-based magical touch attacks work in his game.
The SRD states that you do not, in fact, need to use an action to hold the shocking grasp's charge; casting touch spells is a standard action, but the effect can be activated at any time afterwards, as long as the target is the first creature or object you touch.
To use these spells, you cast the spell and then touch the subject, either in the same round or any time later.
So, you can activate it with the same standard action touch attack roll as with your reserve feat. The only reason that I can see to attempt to discharge the shocking grasp on the same turn as the casting is that you may do so as a free action.
Even if you roll low to-hit, you can keep making reserve feat touch attacks, and your shocking grasp will activate on the first one that hits.