Hi Kitsune,
The reference is to The Listener Crossword No. 4422, which appeared on 29 October 2016. The puzzle was called "Buried Treasure", and was set by Poat.
The theme of the puzzle was the book
Masquerade by Kit Williams, which involved the hunt for a buried golden hare. At the end of the puzzle, solvers were asked to highlight the word HARE in a straight line. However, although the letters appeared several times in the grid, they were never in a straight line. It turned out that the word HARE appeared in a straight line in the preamble, not the grid.
Many solvers thought that this was rather unfair, as those submitting entries are advised not to submit the preamble, just the grid and the name, address and contact details. So, when commentators on the forum refer to "the hare", they are suggesting unfair conduct on the part of the setter.