I agree with unclued that the replacement word begins with a J. Unless my counting skills have failed me even more than usual, the “direction” from the thematic clued answers to their replacements is sometimes forward, sometimes back in the order of the set (eg 1 across vs 10 down). The instruction for the hidden word makes it plain which is the right “direction”.
I felt that the clues for the thematic answers were the hardest of the lot, not helped by having no crossing letters. Some of the sets were obvious, others required hours of speculative internet searching, which took the shine off an interesting puzzle.