This is a lovely self-referential clue for Hopeless at whist, I could get what it is! — the whole thing playfully describes what an ANAGRAM is. The wit lies in the fact that "at whist I" is itself a genuine anagram of "what it is" (both use the letters a, h, i, i, s, t, t, w), so the clue actually demonstrates an anagram while defining the word. Since the answer isn't spelt out from separate wordplay, it works as a cryptic definition.