First letter matches first letter of answer to "Bounder (3)" (which was an assumption at that stage but an obvious one), last letter matches last letter of "Of tailoring (9)" which might not be immediately obvious, but has shown up in this puzzle before (as with the non-GAP answer) and very confirmed by its own cross-links.
(When I was young, I remember reading a book (fiction/adventure, not sure how based upon fact) where some mesoamerican or similar native american civilisation had built canyon-wall citites - bear with me on this, it's getting relevent but not in the way you think! - which, to reach unaided, you needed to climb up or down rock foot-holds carved into the cliffs. The steps were apparently cleverly cut such that only if you stepped into the correct ones with the correct foot at their start, and then progressed onto the more treacherous slope from there the only way you could then continue to step, would you traverse the whole stepped escarpment safely. If you came at it wrong (like an outsider trying to sneak in might do) you'd instead end up in a position where you had been forced to overextend yourself and were now in the dubious position of hanging there, needing to call for help to move on (or back) or fall to your likely death as you tried to move on your own. Aside from the dubious nature of such a set-up, and maybe the memory of the describing passage being a bit rough around the edges as well, it's always in my mind when I start filling in 'obvious' answers in a crossword. What if the setter has deliberately/accidentally made it so that by choosing not-GAP instead of GAP I am 'confirming' other synonyms/alternatives of the real answer, and onward in a chain of 'obvious' choices, but leave myself with some clue almost on the other corner either 'confirmed' with total gibberish or (worse?) a clear wrong answer (such as FIDO instead of ADAM for "First man (4)". No doubt there have been attempts to lay such traps, maybe not across entire crossword grids like that, though. Anyway, probably not in this case, so this aside is probably nothing to worry about.)
I like the thing you're not sold on, BTW, but ditto I don't have much conviction about it. Before the day is out I shall surely have today's puzzles, with yesterday's answers. I rarely check them (occasionally for a word I think I should know, like_A_A_Y for "Whim (6)",and have no more clues to gather) and almost never the cryptic ones, but I shall make a point of checking this 'rocket' one, to see what it is, and maybe working out its actual construction. If I get a satisfactory answer (and 'working out how it is worked out') I shall spoiler it in here for anyone else who is still bothered.
Bear in mind that I'll also then have today's clues to worry about, but if i continue to post those, each day, this thread starts to be Second True Thread-ish, so I shall probably refrain from that!