This does not appear to be a valid inference. The premise tells
us that something in the world fits a certain description, but if
a is the name of a specific individual with which we are already
familiar, we cannot infer that a fits the description.
Nevertheless, it is valid to reason as follows: At least one
thing in the world is both an actor and a waiter. Let's call
that thing a. We can now assert that Aa Wa.
Existential instantiation is valid if we replace the variable
with a name we introduce solely for the purpose of standing for
the particular thing, whatever (or whoever) it may be, that makes
the premise true.
This means that when we use EI in a proof, the name we use must
be one that has not been used previously in the proof and that
does not occur in the conclusion.