Clutch is "coming through in the clutch." Something helpful, especially at a time of distress or just when something is needed. So, if something is clutch, then it is really good because it came through at the right time.
Clutchcakes would also be something cool, but now it has a "sweet" (as in a synonym for cool) connotation to it, hence the 'cake' part of the word. Essentially, the word 'clutchcakes' takes 'clutch' to another level.
One other definition of clutchcakes are extremely good cupcakes that rival Martha Stewart's (as I imagine hers are heaven). They are especially clutch if one gets these extremely good cupcakes at the perfect time. And, one will know when is the perfect time.
So lets say you are at a party and it goes dry, and your friend walks in with a bunch of alki. You can say, that was so clutch! or even better... u can just say.. "clutchcakes." so its better than just being clutch.