Disclaimer: I know little about Access or VBA, but what you're having is a generic Unicode problem, it's not specific to those tools. I retagged your question to add tags related to this issue.
Access is doing the right thing by returning 한, it is indeed the first character of that two-character string. What you want here is the canonical decomposition of this hangul in its constituent jamos, also known as Normalization Form D (NFD), for “decomposed”. The NFD form is ᄒ ᅡ ᆫ, of which the first character is what you want.
Note also that as per your example, you seem to want a function to return the equivalent hangul (ㅎ) for the jamo (ᄒ) – there really are two different code points because they represent different semantic units (a full-fledged hangul syllable, or a part of a hangul). There is no pre-defined mapping from the former to the latter, you could write a small function to that effect, as the number of jamos is limited to a few dozens (the real work is done in the first function, NFD).