Thanks both. Is it me or is it a weird construction? Not sure it quite works logically - a bit like it begs the question.
I see that RLY is an abbreviation of railway, and that RLY is half of hourly... but it feels like there is step missing.
There is nothing showing how you go from RLY to the answer, so you can only work it out once you've guessed at the answer from the definition. I may be overthinking it, but it feels like one of those ones that makes sense once you know the answer, but you'd be hard pressed to work it out without guesswork (or crossers).
Maybe it's that i am expecting a double definition, which it isn't (that would require the answer to be RLY).