"Unclued 18 across and four italicised clues all have two answers. Choose which alternatives to enter according to your personal preference in 6 down and 17 down, and write the one not selected below the clues"
The clue in the OP is one of the italicised clues.
The italicized clues are not cryptic, just straightforward like a quick crossword. You just have to choose one of the two possible answers to fit with the other letters on the grid. Impossible to choose before the rest of the clues are solved.
As for 18 across, I just guessed once I'd solved 6 down and 17 down.