You do need special knowledge of how crosswords work to be able to understand the clue, but that is true of any cryptic crossword clue, not just this one. (I would say that 12a is a particularly recondite clue because of the complexity of its wordplay, but that is by no means a criticism. 19a, on the other hand, has very simple wordplay but is still recondite in the sense that you probably wouldn?