I e-mailed Mike Laws on Saturday about the missing clue but have had no response and I know that at least one other person did the same. It's a waste of a stamp if you happen to enter the wrong one of the four answers! Mike Laws usually responds quickly - I hope he is not ill. I think I'll send him another request.
From a link in another 1164 thread it appears that the clue to 35dn will be printed next Saturday. However it also states that any answer to the single cell remaining will be considered valid.
The possible letters, I think, are d, l, r, u but I suggest that with no clue the most credible answer would be to submit with the single cell left empty as one cannot assume the absence of the clue was not intentional.