That depends, people seem to use some of this terminology interchangeably. Alĺow me to explain the definitions as I use and understand them.
Potentiation - This specifically refers to a synergistic relationship between the substances in question. Example: Alcohol potentiates Benzodiazepines. We know that taking these two substances together creates an additive effect that is greater than the sum of the parts, if you will.
Additive Effect - This is what you´re asking. Does Clonidine (Catapres) produce a synergistic effect when used with Alcohol? No, not that I´m aware of. Clonidine lowers blood pressure and tends to produce a sedative effect, however, this is independent of the effects of Alcohol. Yes, the two taken together are going to produce more total sedation, but this is not the same synergy as described above.
I hope this makes sense. Let me know if you have any questions.