Terry, dunno about "main line" though. But You have spotted it's a "double" albeit quite a cryptic double. I wonder if it merited a "?". Don't think so.
Jolan,
I take a question mark to mean that the answer encompasses the whole clue. Here I took "stretch of main line" to mean the horizon as seen at sea (main). Horizon is a hyponym of reach. Clever,eh,but utter tosh then.
Wow Terry! Not utter tosh at all but very clever indeed. I try my best to avoid ambiguities but sometimes get caught out. Re the "?". I'm never too sure about it's usage (apart from an actual question in the clue). I tend to append one when the clue is particularly tricky but perhaps 'purists' might not agree. However, "we've solved on of the surrounding clues" and now have ???E? for that double.
Chris. Your main = sea interpretation is correct and the word it applies to is also correct LINE. All that's left is to figure out LINE (as referred to at sea) and apply the result to STRETCH and it's solved. Then there's only no. 5.
Although the same exact is not in Chambers, I would accept wake "at a stretch" (LOL, sorry!) but then, what about LINE being singular?.
Okay! We've just "solved another" and now have S??E?