It is not a convention on Answerbank, it has become the convention on the weekly thread that's dedicated to The Listener, but this hasn't always been the case. I've posted hints for the puzzle quite a few times but only in response to specific questions, this is Answerbank, after all.
44a is the thematic entry for an answer of 11 letters, you'll see several other clues where the length given in brackets is more than the space available for the answer - these are all similarly treated (once you get one you'll have good idea how they all work).