13 down - There's mould in his study? Unknown - anyhow, it's gloomy (10).
The answer must be X and an anagram of "it's gloomy".
Grid spaces give ?Y?O?O?I?T. Therefore it should be Xylogomist but there is no such word. The answer could be Xylotomist but this does not tie up with the anagram. Is there an error in the clue?